I've been using 1.5.4 from the upstream .deb package for while. Since Bullseye, fluidsynth is v2, which requires changes that are in the main branch but have never been released, since there hasn't been a release for years.
https://sourceforge.net/p/tuxguitar/patches/39/ I have no idea whether or when upstream maintainers intend to publish a new release. Perhaps Debian could ship a 1.5.4-something to include that patch. I'm afraid I can't do much to help. -- Jérôme
