I don't yet have consistent steps to reproduce this issue, so for now just seeing if others have seen this and if so do know what steps might cause it...
After enabling speed-only varispeed (to decrease tempo to play a difficult part on the keys), I've found that MIDI will sometimes not record. It will behave as if recording (e.g. creating a small blank region), but no notes will record.
Even after turning Varispeed off, it will still sometimes not record.
Through some unknown combination of disarming/rearming the track record, selecting another track and back, and toggling Varispeed, I can eventually get it recording again. But as noted, I haven't found something that consistently clears up the issue; or what consistently causes the issue, though turning on Varispeed seems to be the common element.