I've noticed this as well and glad I'm not the only one seeing this. Before triggering the door, Teebo, QGJ, Fives can reliably land their TM down/slow. After the rancor is toppled and the door comes back up, the first turn or two afterwards I can never land a single one. Occasionally, I see QGJ land a TM down with his basic, but Teebo/Fives are resisted what seems like 100% of the time.
I try to avoid triggering the Door for as long as possible due to this since its nearly impossible to get Slow/TM down applied after he gets up which leads the rancor to start decimating my squad.