On Windows, when using a soundfont other than the default microsoft soundfont, midis will turn all their instruments into pianos when you click outside the ZC window, until you close and restart ZC.
[ZC 2.50.2] Custom soundfont midi turn to piano instruments
Setting midi_patch_fix=1 in ag.cfg should fix that.
People have been reporting this fix didn't do anything for them. For me it did half-work. Clicking out of the screen wouldnt change the instruments for me, but lower the volume a lot. Must be a problem between virtual midi synch and ZC.