Dear Lucas Nussbaum,
I am maintainer from upstream and we just recently changed creation of the .qm files, so this could impact this bug.
See: https://github.com/jamulussoftware/jamulus/pull/3288
While this may be related to debian only, if you believe that we can fix this on our side, please open an issue on our repository too: https://github.com/jamulussoftware/jamulus/issues