Could this have anything to do with ModyPoly key assigner chip / or do you have any idea what could be causing something like this for a single voice? After powering the synth on, the broken voice starts up very high (a couple of octaves or so) and then immediately starts steadily lowering towards the right pitch.
I don’t think this has anything to do with the ModyPoly as the voice assign timing is identical to the original CPU. ut just to be sure I recommend updating the ModyPoly to the latest firmware. See https://tubbutec.de/tubbutec-firmware-updater/