diff --git a/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in b/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in
index e96ba5a1d631..adf14b5af385 100644
--- a/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in
+++ b/python/PyQt6/core/auto_generated/qgsfieldconstraints.sip.in
@@ -72,7 +72,8 @@ is not present on this field.
     ConstraintStrength constraintStrength( Constraint constraint ) const;
 %Docstring
 Returns the strength of a field constraint, or ConstraintStrengthNotSet if the constraint
-is not present on this field.
+is not present on this field. If the strength is not set returns ConstraintStrengthNotSet
+for anything but ConstraintExpression which returns ConstraintStrengthHard.
 
 .. seealso:: :py:func:`constraints`
 
diff --git a/python/core/auto_generated/qgsfieldconstraints.sip.in b/python/core/auto_generated/qgsfieldconstraints.sip.in
index 51ef4a9b8ba8..c9530a1a482c 100644
--- a/python/core/auto_generated/qgsfieldconstraints.sip.in
+++ b/python/core/auto_generated/qgsfieldconstraints.sip.in
@@ -72,7 +72,8 @@ is not present on this field.
     ConstraintStrength constraintStrength( Constraint constraint ) const;
 %Docstring
 Returns the strength of a field constraint, or ConstraintStrengthNotSet if the constraint
-is not present on this field.
+is not present on this field. If the strength is not set returns ConstraintStrengthNotSet
+for anything but ConstraintExpression which returns ConstraintStrengthHard.
 
 .. seealso:: :py:func:`constraints`
 
diff --git a/src/core/qgsfieldconstraints.cpp b/src/core/qgsfieldconstraints.cpp
index c9007529145f..4954916497a3 100644
--- a/src/core/qgsfieldconstraints.cpp
+++ b/src/core/qgsfieldconstraints.cpp
@@ -29,8 +29,8 @@ QgsFieldConstraints::ConstraintStrength QgsFieldConstraints::constraintStrength(
   if ( !( mConstraints & constraint ) )
     return ConstraintStrengthNotSet;
 
-  // defaults to hard strength unless explicitly set
-  return mConstraintStrengths.value( constraint, ConstraintStrengthHard );
+  // defaults to not set for all but expressions (which does not use strength)
+  return mConstraintStrengths.value( constraint, constraint == ConstraintExpression ? ConstraintStrengthHard : ConstraintStrengthNotSet );
 }
 
 void QgsFieldConstraints::setConstraintStrength( QgsFieldConstraints::Constraint constraint, QgsFieldConstraints::ConstraintStrength strength )
diff --git a/src/core/qgsfieldconstraints.h b/src/core/qgsfieldconstraints.h
index a5e0ee5752a8..8bd87ac260ae 100644
--- a/src/core/qgsfieldconstraints.h
+++ b/src/core/qgsfieldconstraints.h
@@ -89,7 +89,8 @@ class CORE_EXPORT QgsFieldConstraints
 
     /**
      * Returns the strength of a field constraint, or ConstraintStrengthNotSet if the constraint
-     * is not present on this field.
+     * is not present on this field. If the strength is not set returns ConstraintStrengthNotSet
+     * for anything but ConstraintExpression which returns ConstraintStrengthHard.
      * \see constraints()
      * \see setConstraintStrength()
      */
diff --git a/src/core/vector/qgsvectorlayer.cpp b/src/core/vector/qgsvectorlayer.cpp
index 7810f5273e64..e41fcc853292 100644
--- a/src/core/vector/qgsvectorlayer.cpp
+++ b/src/core/vector/qgsvectorlayer.cpp
@@ -3145,6 +3145,7 @@ bool QgsVectorLayer::writeSymbology( QDomNode &node, QDomDocument &doc, QString
       constraintElem.setAttribute( QStringLiteral( "unique_strength" ), field.constraints().constraintStrength( QgsFieldConstraints::ConstraintUnique ) );
       constraintElem.setAttribute( QStringLiteral( "notnull_strength" ), field.constraints().constraintStrength( QgsFieldConstraints::ConstraintNotNull ) );
       constraintElem.setAttribute( QStringLiteral( "exp_strength" ), field.constraints().constraintStrength( QgsFieldConstraints::ConstraintExpression ) );
+
       constraintsElem.appendChild( constraintElem );
     }
     node.appendChild( constraintsElem );
diff --git a/tests/src/python/test_qgsvectorlayer.py b/tests/src/python/test_qgsvectorlayer.py
index 881578a0bb0f..c6b4918d00a1 100644
--- a/tests/src/python/test_qgsvectorlayer.py
+++ b/tests/src/python/test_qgsvectorlayer.py
@@ -4675,6 +4675,47 @@ def test_selection_properties(self):
             self.assertEqual(vl2.selectionProperties().selectionColor(),
                              QColor(255, 0, 0))
 
+    def testConstraintsotSet(self):
+        """Test that a NotSet constraint does not become a Hard constraint
+           when saving and loading a layer style, see issue GH #58431"""
+
+        # Create a memory layer with unique constraints
+        layer = QgsVectorLayer("Point?field=fldtxt:string&field=fldint:integer", "test_unique", "memory")
+        self.assertTrue(layer.isValid())
+
+        # Se not constraints on fldtxt
+        layer.setFieldConstraint(0, QgsFieldConstraints.Constraint.ConstraintNotNull, QgsFieldConstraints.ConstraintStrength.ConstraintStrengthSoft)
+        layer.setFieldConstraint(0, QgsFieldConstraints.Constraint.ConstraintUnique, QgsFieldConstraints.ConstraintStrength.ConstraintStrengthNotSet)
+
+        # Check constraints at the field level
+        field = layer.fields().at(0)
+        constraints = field.constraints()
+        self.assertEqual(constraints.constraintStrength(QgsFieldConstraints.Constraint.ConstraintNotNull), QgsFieldConstraints.ConstraintStrength.ConstraintStrengthSoft)
+        self.assertEqual(constraints.constraintStrength(QgsFieldConstraints.Constraint.ConstraintUnique), QgsFieldConstraints.ConstraintStrength.ConstraintStrengthNotSet)
+
+        # Check constraints at the layer level
+        constraints = layer.fieldConstraintsAndStrength(0)
+        self.assertEqual(constraints[QgsFieldConstraints.Constraint.ConstraintNotNull], QgsFieldConstraints.ConstraintStrength.ConstraintStrengthSoft)
+        self.assertEqual(constraints[QgsFieldConstraints.Constraint.ConstraintUnique], QgsFieldConstraints.ConstraintStrength.ConstraintStrengthNotSet)
+
+        # Export the style to QML and reload it
+        style = QgsMapLayerStyle()
+        temp_file = tempfile.mktemp(suffix='.qml')
+        layer.saveNamedStyle(temp_file)
+        layer.loadNamedStyle(temp_file)
+
+        # Check constraints at the field level
+        field = layer.fields().at(0)
+        constraints = field.constraints()
+        self.assertEqual(constraints.constraintStrength(QgsFieldConstraints.Constraint.ConstraintNotNull), QgsFieldConstraints.ConstraintStrength.ConstraintStrengthSoft)
+        self.assertEqual(constraints.constraintStrength(QgsFieldConstraints.Constraint.ConstraintUnique), QgsFieldConstraints.ConstraintStrength.ConstraintStrengthNotSet)
+
+        # Check constraints at the layer level
+        constraints = layer.fieldConstraintsAndStrength(0)
+        self.assertEqual(constraints[QgsFieldConstraints.Constraint.ConstraintNotNull], QgsFieldConstraints.ConstraintStrength.ConstraintStrengthSoft)
+        self.assertEqual(constraints[QgsFieldConstraints.Constraint.ConstraintUnique], QgsFieldConstraints.ConstraintStrength.ConstraintStrengthNotSet)
+
+
 # TODO:
 # - fetch rect: feat with changed geometry: 1. in rect, 2. out of rect
 # - more join tests