Trissa Whistlesong
 

Mob spawns in proper spot, static spawn until engaged.

When engaged, it appears that (2) rounds of combat occur as normal. After that, she *pops* out of the room to the second floor.

She's still able to engage you from there (I have logs to show it).
You are not able to hit her - getting two errors:

You can't it them from here.
Your target is too far away, get closer!

If you wait, she comes back to re-engage properly but she is still technically hitting you the entire time.

Upon pathing back and re-engaging, (2) more rounds then she *pops* back up to the second floor.

If I FD, she paths back to her normal spot. Any attempts to re-engage continue the *pop* cycle.

Have not tried rooting to see if that pins her to her spawn - will try so with another character and see if that changes anything.