Jeremiah Pierson
Inspired
This same issue happened to me. I added a volume block into a preset. Calibrated the expression pedal (plugged into the FM3 itself) and assigned it. When I go to swell in slowly, the volume sweep goes about 2/3 the way up the slope (looking at it in the modifier window) with virtually no change in volume (ie silent) and then the volume jumps to on and continues to increase in volume smoothly the rest of the sweep. Same whether using my modded Ernie Ball VP Jr or my Dunlop Volume x Mini. Once at the toe position, the volume sweeps smoothly down and back up until I rest the expression pedal at heel and try again. It sounds like there is a noise gate threshold in the volume block that is clicking open at some threshold and then not closing as long as signal is passing. Hope that’s enough detail.
I had a reverse polarity cable that caused this same behavior. I used reverse polarity cables for my Mastermind PBC and my EV-2 pedal and took me forever to figure out the problem. Test your cable to make sure it's tip to tip, ring to ring, and sleeve to sleeve. If the tip and ring are swapped you have a reverse polarity cable and replace it with a standard TRS cable.