Spent the morning trying to reproduce by deliberately shorting some things but the symptom did not occur with top off.
Added some “Contact paper” insulation in two places where it could have been and put just the top on and tested. Shutter is responding as it should.
Tried looking at the third body and it looks like it needs thorough diagnostics so I just put that one together and left myself a note.
Back to the “bad” one and it’s still working so I put it all the way together. It continues to work, and I checked a couple times since. Still OK.
Basically this bad one is a “donor body” that’s had lots of good stuff swapped out of it, but sewn back up and happens to work.