My issue with the mysterious sustain is resolved.
Copy/pasted from another post on what I initially thought was a separate issue:
I had a section of MIDI that left a hold CC at 100. Of note, this section did not have to be played for it to effect a section of MIDI that came after it. This didn’t line up with my intuition of how CC triggers on a track worked and thus my difficulty locating the problem. My notion was if you created a fresh measure of MIDI, put the start marker at that measure and hit play that measure would not be effected by anything coming before it. It appears Cubase prescans the track for CCs however, which I actually think is great for when tweaking individual parts of a measure, but can make troubleshooting a little rough if you don’t know this is how it works.
I wish keyswitches could be prescanned in the same way. Working with individual measures in Ample Sound Guitars for instance can be sort of a pain since some keyswitches have to occur immediately before the measure. Thus if you are looping a measure while tweaking it you may be missing an important keyswitch. Hmm, I wonder if expression maps are prescanned. Will have to test.