I used some silicon lube from a spray can applied with a paper towel onto the O-ring and threads. It seems less viscous than whatever was already on there. The result is just that it’s easier to screw all the way, but doesn’t change the finicky behavior. If anything it seems I really need to screw it down all the way now, since with the O-ring more lubricated, it allows the head to move more easily.

I suspect this is just due to play in the threads. The head has to be firmly screwed in so that it is seated onto the ledge in the tube, otherwise I guess it can rock. The contacts are on the bottom surface of the PCB touching the ledge, so this can cause them to make/break the connection, causing flickering or cycling through modes. At least that’s my current hypothesis. I imagine this is why contacts are often designed as springs.