Will you tell us all the knob values on your preset that has this problem, as well as expression pedal settings? I think that might be a good starting place. To me it really does sound like the modulation is set to Square wave, and the depth is set to some negative interval. Either that or the expression pedal is mapped to do this drop and something (possible bug) is triggering expression pedal somehow. But we really need more info about you settings and setup to help diagnose the problem.