Zombie Mode Bosses Bugs

Recently we played some Crater and Venice games, the bosses where disappearing for 1-2 seconds and later they appeared again. This happened several times before they die.
Then yesterday we had as daily Final Arena, the boss Corrupted Engineer before it attack some one it started to spin in the same place.
Today in Devastated city at round 20 the boss also started to disappear and appear.
What's wrong ? :confused:

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