From the release notes for u23.3:
Players will no longer be able to retreat while their Fellowship is engaged in combat with a boss monster. Players can still accept revives from other players.
This is bugged where sometimes you can't retreat even after the encounter ends and the boss resets. Just had a raid where after boss 4 wiped us by the grims, no one was able to retreat. Dropping group didn't autoretreat us after 20s or even after 2 minutes, and we couldn't log out. The only way for us to leave the instance was to force-crash LOTRO and log back in, and reform the raid. Why is this a thing? Just revert this annoying change.