I am controlling the H9 via MIDI with a Line 6 HX effects, and I have the same problem from time to time. Volume drop, unless I reboot the pedal.
When that happened first, I tried isolating the pedal (Guitar in, Amp Out0, no Midi cable plugged in, and the volume drop was still there until I rebooted it.
Did you ever figured it out?
Any answer would be greatly appreciated 🙂