I will fill this in more on Monday, just opening the ticket now before I forget. I now have a minimal repro in Swift of this but have not gotten a chance to translate it to equivalent C code.

The gist is that since upgrading to libmongoc 1.17.0 in the Swift driver we have been experiencing somewhat frequent failures of our transactions tests on macOS. Locally I am now finding that in particular this test fails for me about 90% of the time:
https://github.com/mongodb/specifications/blob/1aa2135d930a6230b7469c02377c33e121f9719e/source/transactions/tests/mongos-recovery-token.yml#L178

Specifically, when destroying the test MongoClient (which is backed by a client pool), there seems to be a crash while calling pthread_join to shut down the server monitor threads, and the Swift program exits with signal code 13.