Hello it seems that alot of bosses get frozen. then you have to jump from channel to channel to find one that is not frozen.
Well i think I might found the reason for this bugg.
When I was fighting a boss and I died the boss froze in place this also happens to alot of ultra and super too, but it doesnt afect normal mobs because when u die they reset them selfs. but when boss mob kills you it doesnt seem to fully reset back to place but freezes in place instead.
I have done this several times already with bosses where I die and they instally froze. there a chance for ultra and super to freeze but never happened with normals.
Hope this helps out.