Thank you for testing! I'll see if I can find a simple solution to the missed notes.
Edit: it is very likely a bug in the fluidsynth patch for dosbox instead of fluidsynth itself. Confirmed by running the native fluidsynth binary here: Converting MIDI output to an audio file, which produces the proper instrument samples while using robust soundfonts. My best guess is that there is a problem in processing a midi event because the patch is incomplete. I wish I could isolate the midi events which correspond to the missing percussion sounds.
Also, I now recall this "missing instrument" issue while working on the patch, but I didn't debug beyond examining the original code for obvious errors in the workflow. There is an alternative patch here: FluidSynth MIDI driver, but it didn't compile cleanly, at least initially, so I used the above patch. In addition, both patches appear very similar, although with this missing instrument issue, it pays to try the alternative patch and confirm whether it somehow avoids this issue. Each handles the midi processing with a different API set (of functions), too.
Edit2: isolated the issue. It's the selected drum set. Confirmed by using "megamid" software in dosbox; the drums are missing unless the drum kit is changed by pressing the "." key. My best guess is that the drum kit isn't finding a proper default value upon playback of the midi file.