Mainstage MIDI In issues w/old devices

I bought a Roland HP-800 digital piano with MIDI DIN cables connected to my mac going to mainstage. When I play notes, as soon as I let go of all of the notes the sound cuts out immediately, with the small pop when cutting the middle of audio. When playing with Logic Pro's MIDI, this doesn't happen. The sound doesn't cut immediately.

I've been dealing with this issue for a while now, and I can't find a solution. I've had an audio technician0 who has worked with mainstage come over and not be able to figure it out.