The sf2 soundfont mappings in the sampler is completely broken in 1.29.13.0, it is also broken in 1.29.12.0, the grand piano sounds like some sci-fi sound and the drums don't make any sounds, I thought this stuff was fixed, it is worse then ever. I'm running windows 10 64 bit Version 21H1, I'm using GeneralUser GS v1.471.sf2.
Could this be a windows bug messing up OpenMPT?
Windows can be blamed for many things but not for the poor compatibility between how soundfonts work and how OpenMPT's internal sampler works. Simply put: The probability that there is a bug in Windows so bad that it makes OpenMPT extract the wrong sample from a soundfont but otherwise all applications keep working is next to 0. You can stop mentioning this every time you discover a bug in OpenMPT.
The problem only seems to appear when putting the soundfont into OpenMPT's MIDI library, not when loading it as a stand-alone soundfont via the "Add Sound Bank" item. This hints at the MIDI library code looking for drum instruments where it shouldn't, because the sound you're hearing is a drum kit sound rather than a melodic sound.
Last but not least, if you need good SF2 support, I'd recommend using a VST plugins such as Sobanth or Sforzando, because those are unlikely to have this sort of bugs.
r15899 (which will probably be backported to OpenMPT 1.29) fixes various issues encountered with that particular soundfont (and probably other more advanced soundfonts). All issues that I could find are fixed, but don't hold your breath - there are probably more, and they're not Windows bugs.
Has r15899 been added to Master branch on Github?
Yes, our GitHub mirror is updated automatically. In the meantime the fixes have also been backported for the next OpenMPT 1.29 release.
The update is now available: https://openmpt.org/openmpt-1-29-14-00-released