This is a known issue, from the comment, but it is causing AVs for my embedded application when background threads are created, for example, by the _socket builtin module.

/* Undo the effect of Py_Initialize().

   Beware: if multiple interpreter and/or thread states exist, these
   are not wiped out; only the current thread and interpreter state
   are deleted.  But since everything else is deleted, those other
   interpreter and thread states should no longer be used.

   (XXX We should do better, e.g. wipe out all interpreters and
   threads.)

   Locking: as above.

*/

void
Py_Finalize(void)

I'm closing this in favor of #36476 "Runtime finalization assumes all other threads have exited."