The 11/70 suppresses the recognition of break requests at the end of a vector flow of a stack error abort. The processor manual states
6.2.2.3 Timing of Stack Error Aborts
BRQ STROBE is thus inhibited, not only during ZAP.00, but also during
SVC.90, thus guaranteeing the execution of the first instruction of the
error subroutine before any other error can be processed.
The rationale behind was apparently to ensure that the handler, which
is called with SP=0 pointing to an emergency stack, can set up a new
stack before any other condition interferes.
SimH does not implement this behavior, nor does w11. Beyond that, it would be very difficult to verify this extreme corner case.