Running tests in parallel on Windows quits too soon

On my Win10 the test suite completes when run serially. But with main and 3.11, but not 3.10, it quits too soon with -j0. This has occured with both repository debug builds and installed 3.11.0b4. Failure is currently deterministic with variable details. Presence of -ugui or -uall has no apparent effect.

What happens is that roughly about 100 tests before the end, a 'regrtest worker thread' fails ('warning') with UnicodeDecodeError: 'utf-8' codec can't decode byte 0x91 in position <variable>: invalid start byte. The 14 worker processes are stopped and the usual summary is given, but with an additional list of ' tests omitted <list of test names'. The test is called a 'SUCCESS'. This is followed by a traceback for SystemExit(0), followed by 1 or more tracebacks for PermissionError because a temporary test file is supposedly used by another process.

Attaching a file with output starting with the initial warning fails. Will paste separately.

If this is not limited to my system, I think it should be a release blocker.