I think I found the answer (after switching to Bitwig).
LMMS from the Ubuntu repository uses the Fluidsynth library. When I use it with Carla, it sounds similar. The AppImage, probably, have a lighter or simpler library, that's why there is the difference.
LMMS from the Ubuntu repository uses the Fluidsynth library. When I use it with Carla, it sounds similar. The AppImage, probably, have a lighter or simpler library, that's why there is the difference.