Does the same happen if you hit home first adjust the volume then return to the emu

I don't have a Go myself to test anything with, but I do know the firmware although the version indication is the same the actual firmware updates are different to the normal PSP 1k 2k and 3k updates and obviously has taken effect with that one emulator

Pro C with fix 3 applied may be worth trying though

Pro C-2 breaks official PSone classics though