There were PvE combat changes a while ago where players joining late can still act in that round, and other would not be able to join if all enemies will die that turn. No monsters could join either.
Now, as I noticed recently, mobs can now once again join the combat even if all other mobs currently in the fight would die that round.
It feels absolutely awful as I loved this change! I was mostly doing solo farming but the last turn lock should still be in play even if one is solo, and it should also prevent monsters from joining not just players. If the battle is already over and I'm just waiting for animations, I don't want to spend time on a prolonged fight but take the rewards and start a new one instead.
Please, if it's a bug, look into ways of fixing it, and if it was intentionally reverted back, change it. It's not good.