This appears to have started once gh-104210 was merged.

"Fatal Python error: drop_gil: PyMUTEX_LOCK(gil->switch_mutex) failed"

It may be specific to test.test__xxsubinterpreters.RunStringTests.test_create_thread.

See:

• https://buildbot.python.org/all/#builders/484/builds/3261
• GH-104308: socket.getnameinfo should release the GIL #104307 (comment)

Linked PRs

• gh-104341: Ensure tstate Holding GIL is Always Current #104436
• gh-104341: Adjust tstate_must_exit() to Respect Interpreter Finalization #104437
• gh-104341: Clean Up threading Module #104552
• gh-104341: Fix threading Module Shutdown #104560
• gh-104341: Minor Fixes in the _thread Module #104595
• gh-104341: Wait Completely at threading._shutdown() #104672
• [reverted] gh-104341: Add a Separate "Running" Lock for Each Thread #104754
• [3.12] gh-104341: Add a Separate "Running" Lock for Each Thread (gh-104754) #104817
• gh-104341: Call _PyEval_ReleaseLock() with NULL When Finalizing the Current Thread #105109
• [3.12] gh-104341: Call _PyEval_ReleaseLock() with NULL When Finalizing the Current Thread (gh-105109) #105209