If an enemy has the text "cannot leave the staging area," loses that text, is engaged, and then regains that text, does it bounce back to the staging area?
I'd assume so.
I just had a narrow loss against The Black Serpent. I had 6 progress on stage 3. The Black Serpent enemy has the text "cannot leave the staging area," but he loses that text if you have 6 progress on the quest stage. I had to endure the Serpent's Forced effect attack, to avoid losing progress on the quest stage, so I could engage him. I did that, but I couldn't handle another attack from him, so I didn't engage. During combat, Tactics Aragorn killed another enemy, so I could engage the Serpent through Aragorn's effect (he is never immune to player card effects). But all of my attackers were exhausted, so I couldn't damage him.
There was an unattached Ambush in the staging area. I also had the "Don't Let them Escape!" side quest in play, which meant all of the main quest's progress was going to get wiped at the end of the round. I assume this means the Serpent would bounce back. That's where I'm not clear. If he bounced back, he would be snared in that Ambush, making for an easy victory next turn as long as I put down 6 progress and survived until the combat phase. Unfortunately, I had 49 threat in the Aragorn deck and I lost by threating out before I could make this work. Anticlimactic... Good game though.