Vexes don't take damage after a period of time

Steps to reproduce

1. Set up a repeat command block with a teleport command to keep vexes trapped in a particular spot.
2. Start a raid on hard difficulty.
3. When you get to a wave with evokers, allow them to spawn vexes.
4. Do not kill the vexes, but kill the rest of the raid.

Expected result

Vexes spontaneously start taking damage at a rate of 1 health/second after existing for 30-119 seconds.

Observed result

Vexes never spontaneously start taking damage.

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After a raid that includes evokers is long over, any vexes that were summoned will continue to hang around, becoming aggressive if approached, but otherwise bobbing up and down and doing nothing else.