A note for the future. This wasn't actually due to differences in architectures, but due to upstream not escaping \ in docstrings, which swig was then replacing with garbage. I originally fixed this by separating out the fltk.py for different architectures, but that wasn't the problem, so I reverted that. The actual fix is here:
https://salsa.debian.org/python-team/packages/pyfltk/-/blob/master/debian/patches/swig-docstrings-escape-backslash.patch
