Please provide an example soundfont you see the problem happening with, and explain what you are doing with it. I see no problems with soundfonts here.

PS: Please do not modify the priority of an issue. Just because a feature that may not be working fully is important to you, doesn't mean that it will our top priority to fix it.

In this thread some soundfonts have been mentioned: 4GMGSMT.SF2, GeneralUserGS144.sf2 and GeneralUser GS v1.471.sf2

I can reproduce the issue with the GeneralUser soundfont but not with e.g. the Yamaha XG soundset or GM.DLS.

Problem was introduced in OpenMPT 1.29. Potentially caused by support for global zones.

Any way to fix this with soundfonts like the Ultimate Genesis Soundfont?

Eventually, sure. I did have a look at it earlier this week and couldn't come up with a fix yet. Soundfonts are complicated.

As a temporary workaround you could convert those soundfont instruments to individual instruments (e.g. SFZ) with a program like Awave Studio and then assign those to the MIDI library.

This was caused by support for global regions that was introduced in OpenMPT 1.29. r15475 fixes this and will be part of the next OpenMPT update.