In his second phase when he summons all screen orbs, if the orbs are destroyed too quickly, high chances are he just stand there and do nothing when he recover, u can just kill him without any retaliation. Or he may attack a bit, but after change to phase 1and change back, he will be still, with some lingering tenticles in the sky.
#Annihilation Phase2 standing still bug
dull portal
Also not related to the bug mentioned above, games seems to crash a lot in anni or venom fight, which doesn't seem to happen when playing other worlds.
thorny comet
Seen this happen a ton of times using song of eafir to wipe orbs.
He just sits there.
hexed pier
added to list
pearl mauve
I used to have a nearly 100% success rate reproducing this by standing about 25% of the map length from the boss when he's winding up to spawn the orbs. Doing that puts you right next to the orb that spawns near the floor in the center of the room. Then using Song of Eafir while looking straight down as soon as the stagger ended, you may or may not need Amplitude for this but I used it