I don't know anything about repairing DMDs, but I was reading other forums/sites like https://www.pinballmedic.net/, etc and might have an idea.
If the issue follows the DMD through multiple machines, it might be a DMD connector somewhere on the DMD itself.
I saw a video somewhere of a guy who had a DMD which was showing a few missed dots, but still played videos/pictures. He pushed the board connector in further and the issue went away, good as new.
Thinking along these lines, you can try and look at the DMD and see if the wires are securely connected inside the connectors themselves.

Hopefully this helps!