The most obviously relevant change between those is that the installed BLAS changed from atlas to openblas.
It looks like this wasn't a change of default, but that experimental uses libatlas and unstable uses libopenblas.
(I don't know why, as the obvious dependency chain is src:statsmodels -> python3-scipy -> python3-pythran -> libopenblas-dev | libatlas-base-dev on both, and libopenblas is installable in experimental.)
It looks like version 0.3.21+ds-4 is not affected by this issue and passes its testsuite. Can you possibly check whether statsmodels builds against that version?
It did (successfully) in statsmodels version 0.13.5+dfsg-7. The openblas build logs also show the issue starting between .21 and .22.
