When killed by a mob multiple times all mobs of that type will turn invisible, this will only happen when there are alot of command block running (of a 20Hz clock) Cannot recreate in world without the command block clocks
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More like MC-68403 since it involves dying and respawning.

... but the mobs die and respawn. MC-68403 is about player dying and respawning...

When killed by a mob multiple times
This seems to be talking about players being killed by mobs (unless I'm missing something here); also I don't think mobs can respawn in Vanilla Minecraft xD.

You're right.
Dupe of MC-65040