@ZOS_GinaBruno
Now that the festival is over I have the chance to solo dolmens and I discovered that the new mobs like grievous twilight, flesh colossus, etc. don't give xp or loot when killed. This holds true even when it's an unnamed boss like the dread flesh colossus (I tested on 3 different dolmens before posting, with 2 different new mob bosses).
I've noticed this on my first dolmen after One Tamriel went live yet I wasn't sure if the large group of players were doing more damage then me, preventing XP and loot but now I'm certain it's not the case.
I'm on Xbox NA if it matters.