Very useful information, thanks!!
and another fun observation: when the instrument gets in this state, NRPN transmission is also jumbled. Here's changing duration in a paraphonic sequence:
23:07:12.007 From HST 2 Control 1 B0 26 5F
23:07:12.007 From HST 2 Control 1 B0 63 08
23:07:12.007 From HST 2 Control 1 B0 62 6B
23:07:12.007 From HST 2 Control 1 B0 06 00
whoops!
I've been following up with support, but wanted to follow up here, as well. In the meantime, I can reproduce without sysex communication, without paraphonic mode, without much of anything, just by turning on the synth and mashing keys for a while.
Yesterday, I was able to reverse the problem by unplugging & replugging the USB cable. After doing that, I couldn't make the problem re-occur. After doing a power cycle, I could make it happen within a few minutes again. That was great, but... it didn't work today.
Today, the reversal also affected the internal sound, which never happened before. USB cable plugstuff didn't have any effect.
In any case, there's something wrong and it's truly annoying. By all means reach out if there's anything I can do to help you reproduce the issue.