Zombies that don't despawn, such as zombies with items in their hands or zombies in the igloo dungeons, become invisible. I'm not sure when, i think when the chunks get unloaded. When i close the game and re-enter, the zombies are visible again.
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Comments 2
Duplicate of MC-193509. This is because you are using Optifine
Please set this issue as invalid as it's probably an Optifine issue.