In FluidSynth 2.2.0 an extra state was added to denote stopping. To transition
from this state to a stopped state the rendering needs to be running. Since
04ce1f07 locking was added that skipped the rendering when something else held
a lock, so the state would never get to stopped and join would never return.
Debian now provides a default soundfont for FluidSynth via its alternatives system.
In short, FluidSynth is configured to look for `/usr/share/sounds/sf3/default-GM.sf3` as its default soundfont, and each soundfront package (FluidR3, OPL-3, MuseScore...) may provide or override this symlink. By default, FluidSynth is installed on Debian with the `TimGM6mb` soundfont by default due to its limited size.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=929185 for further details.