FWIW, this is the same munit bug found in nghttp3: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1097462
Note that the munit code shipped there does not come from upstream, where the PR with this fix is open for a while ( https://github.com/nemequ/munit/pull/105), but a fork where the relevant change was applied: https://github.com/ngtcp2/munit/commit/083b619c45aff90cf815579d83f1a9b1d5131d1d . OTOH in nghttp2 the munit submodule code is being explicitly added as a patch; seems that patch needs to be updated to include the commit above.
