That sounds like an oddball thing to happen but let's try a few things with the most obvious before we jump to any conclusions.
1). Check each MIDI cable to each unit in the chain with a known good cable to be sure that none of them are failing. Try several program changes with the replacement cable to be certain it's functioning like it should.
2). If that doesn't correct the issue, then check the MIDI channels in on each MIDI unit in the chain to verify they are transmitting and receiving on the channels they are supposed to. Hard reboots can sometimes cause MIDI systems to refer to default which is commonly OMNI mode.
MIDI Continuous Control messages will be ignored by the MP-1, but will be passed through to the next MIDI device unaltered. The MP-1 has no CC functions but will respond to program change messages on it's assigned channel.
Another thing you can try, although this seems like it wouldn't make any difference, I have experienced this with my Ground Control when I had multiple MIDI units in the chain, and each one was on a different channel:
If the Sequencer transmits to multiple units on the same chain, but with independent channels for each unit, then try to assign the channels to either odd or even number channels on the chain. For example:
If the 1st MIDI piece of hardware is assigned to MIDI channel 1, then the next piece should be on MIDI channel 3, and so on...
With certain MIDI hardware, I had experienced unresponsive or mistaken MIDI commands when the MIDI channels were consecutive. It wasn't a consistent problem, it happened randomly, but that helped my issue out.
I hope this gives you something to go on.
Harley