Have tried variations of f5, mixer, editor. always come back to same instrument.
I duplicate your finding checking on your file 'test1.mgu'.
Changes of patches do not work whenever the midi signal is piped through some VST/DXi synth.
Changes of patches do work when such a synth is not acive and e.g. the Wavetable synth is active instead.
This issue may be a ticket for PGMusic Support to be resolved . . .