@crusaderky tracked down a thread safety issue over in SciPy to code generated by f2py. See scipy/scipy#21936 (comment).

Lo and behold, there's a static struct I missed when I went over the f2py codebase last year:

I think the simplest fix is probably to make the struct thread-local. @crusaderky told me they were working on an isolated test case to trigger the issue they hit and a patch.