When submerged in water zombies turn invisible (not a potion effect). In the even that they are in 1 or 2 blocks of water the part of their body above water is still visible but the submerged part is not.
In the provided image you can see the tops of the zombies heads however the rest of the is invisible.
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If I see it happen again I will. I've only seen it happen once or twice and this was the first time I thought to report it. Also the error did not reappear with other zombies when I attempted to replicate it (I did not relog at any point).
@Garfieldwxg55 I was able to recreate the glitch and got the f3 screen shot.
Can you provide a debug (F3) screen of the glitch occuring?