IMO, this is a trick these criminal MMs use to kill momentum. They stick themselves on the offer and don't fill. So after a while the system recognizes they are stuck and allows it to print above the stuck MM(s)....but the stuck MMs still present a blockade for it moving up.

Interestingly, I see this happen only when the PPS is trying to move up. There's never any stuck MM on the bid preventing it from going down.

Shenanigans at their finest.