diff --git a/singer_sdk/helpers/_flattening.py b/singer_sdk/helpers/_flattening.py
index 397044bb6..2e239d9d0 100644
--- a/singer_sdk/helpers/_flattening.py
+++ b/singer_sdk/helpers/_flattening.py
@@ -185,6 +185,101 @@ def flatten_schema(
         }
       }
     }
+
+    >>> nullable_leaves_schema = {
+    ...     "type": "object",
+    ...     "properties": {
+    ...         "id": {
+    ...             "type": "string"
+    ...         },
+    ...         "foo": {
+    ...             "type": ["object", "null"],
+    ...             "properties": {
+    ...                 "bar": {
+    ...                     "type": ["object", "null"],
+    ...                     "properties": {
+    ...                         "baz": {
+    ...                             "type": ["object", "null"],
+    ...                             "properties": {
+    ...                                 "qux": {
+    ...                                     "type": "string"
+    ...                                 }
+    ...                             }
+    ...                         }
+    ...                     }
+    ...                 }
+    ...             }
+    ...         }
+    ...     }
+    ... }
+    >>> print(json.dumps(flatten_schema(nullable_leaves_schema, 0), indent=2))
+    {
+      "type": "object",
+      "properties": {
+        "id": {
+          "type": "string"
+        },
+        "foo": {
+          "type": [
+            "object",
+            "null"
+          ],
+          "properties": {
+            "bar": {
+              "type": [
+                "object",
+                "null"
+              ],
+              "properties": {
+                "baz": {
+                  "type": [
+                    "object",
+                    "null"
+                  ],
+                  "properties": {
+                    "qux": {
+                      "type": "string"
+                    }
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+
+    >>> print(json.dumps(flatten_schema(nullable_leaves_schema, 1), indent=2))
+    {
+      "type": "object",
+      "properties": {
+        "id": {
+          "type": "string"
+        },
+        "foo__bar": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      }
+    }
+
+    >>> print(json.dumps(flatten_schema(nullable_leaves_schema, 2), indent=2))
+    {
+      "type": "object",
+      "properties": {
+        "id": {
+          "type": "string"
+        },
+        "foo__bar__baz": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      }
+    }
     """
     new_schema = deepcopy(schema)
     new_schema["properties"] = _flatten_schema(
diff --git a/tests/snapshots/mapped_stream/flatten_all.jsonl b/tests/snapshots/mapped_stream/flatten_all.jsonl
index 9a1b53b5b..e588c9dc1 100644
--- a/tests/snapshots/mapped_stream/flatten_all.jsonl
+++ b/tests/snapshots/mapped_stream/flatten_all.jsonl
@@ -1,5 +1,5 @@
 {"type": "STATE", "value": {}}
-{"type": "SCHEMA", "stream": "mystream", "schema": {"properties": {"email": {"type": ["string", "null"]}, "count": {"type": ["integer", "null"]}, "user__id": {"type": ["integer", "null"]}, "user__sub__num": {"type": ["integer", "null"]}, "user__some_numbers": {"type": "string"}}, "type": "object"}, "key_properties": []}
+{"type": "SCHEMA", "stream": "mystream", "schema": {"properties": {"email": {"type": ["string", "null"]}, "count": {"type": ["integer", "null"]}, "user__id": {"type": ["integer", "null"]}, "user__sub__num": {"type": ["integer", "null"]}, "user__some_numbers": {"type": ["string", "null"]}}, "type": "object"}, "key_properties": []}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "alice@example.com", "count": 21, "user__id": 1, "user__sub__num": 1, "user__some_numbers": "[3.14, 2.718]"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "bob@example.com", "count": 13, "user__id": 2, "user__sub__num": 2, "user__some_numbers": "[10.32, 1.618]"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "charlie@example.com", "count": 19, "user__id": 3, "user__sub__num": 3, "user__some_numbers": "[1.414, 1.732]"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
diff --git a/tests/snapshots/mapped_stream/flatten_depth_1.jsonl b/tests/snapshots/mapped_stream/flatten_depth_1.jsonl
index ab5f3a30b..e3f6cd05f 100644
--- a/tests/snapshots/mapped_stream/flatten_depth_1.jsonl
+++ b/tests/snapshots/mapped_stream/flatten_depth_1.jsonl
@@ -1,5 +1,5 @@
 {"type": "STATE", "value": {}}
-{"type": "SCHEMA", "stream": "mystream", "schema": {"properties": {"email": {"type": ["string", "null"]}, "count": {"type": ["integer", "null"]}, "user__id": {"type": ["integer", "null"]}, "user__sub": {"type": "string"}, "user__some_numbers": {"type": "string"}}, "type": "object"}, "key_properties": []}
+{"type": "SCHEMA", "stream": "mystream", "schema": {"properties": {"email": {"type": ["string", "null"]}, "count": {"type": ["integer", "null"]}, "user__id": {"type": ["integer", "null"]}, "user__sub": {"type": ["string", "null"]}, "user__some_numbers": {"type": ["string", "null"]}}, "type": "object"}, "key_properties": []}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "alice@example.com", "count": 21, "user__id": 1, "user__sub": "{\"num\": 1}", "user__some_numbers": "[3.14, 2.718]"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "bob@example.com", "count": 13, "user__id": 2, "user__sub": "{\"num\": 2}", "user__some_numbers": "[10.32, 1.618]"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "charlie@example.com", "count": 19, "user__id": 3, "user__sub": "{\"num\": 3}", "user__some_numbers": "[1.414, 1.732]"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
diff --git a/tests/snapshots/mapped_stream/map_and_flatten.jsonl b/tests/snapshots/mapped_stream/map_and_flatten.jsonl
index a82a124f7..921094d05 100644
--- a/tests/snapshots/mapped_stream/map_and_flatten.jsonl
+++ b/tests/snapshots/mapped_stream/map_and_flatten.jsonl
@@ -1,5 +1,5 @@
 {"type": "STATE", "value": {}}
-{"type": "SCHEMA", "stream": "mystream", "schema": {"properties": {"email": {"type": ["string", "null"]}, "count": {"type": ["integer", "null"]}, "user__id": {"type": ["integer", "null"]}, "user__sub__num": {"type": ["integer", "null"]}, "user__some_numbers": {"type": "string"}, "email_hash": {"type": ["string", "null"]}}, "type": "object"}, "key_properties": ["email_hash"]}
+{"type": "SCHEMA", "stream": "mystream", "schema": {"properties": {"email": {"type": ["string", "null"]}, "count": {"type": ["integer", "null"]}, "user__id": {"type": ["integer", "null"]}, "user__sub__num": {"type": ["integer", "null"]}, "user__some_numbers": {"type": ["string", "null"]}, "email_hash": {"type": ["string", "null"]}}, "type": "object"}, "key_properties": ["email_hash"]}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "alice@example.com", "count": 21, "user__id": 1, "user__sub__num": 1, "user__some_numbers": "[3.14, 2.718]", "email_hash": "c160f8cc69a4f0bf2b0362752353d060"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "bob@example.com", "count": 13, "user__id": 2, "user__sub__num": 2, "user__some_numbers": "[10.32, 1.618]", "email_hash": "4b9bb80620f03eb3719e0a061c14283d"}, "time_extracted": "2022-01-01T00:00:00+00:00"}
 {"type": "RECORD", "stream": "mystream", "record": {"email": "charlie@example.com", "count": 19, "user__id": 3, "user__sub__num": 3, "user__some_numbers": "[1.414, 1.732]", "email_hash": "426b189df1e2f359efe6ee90f2d2030f"}, "time_extracted": "2022-01-01T00:00:00+00:00"}