Vex could sometimes be seen chasing the player without returning to the summoner, repeating his angry streak. If the player gets too close to the Vex with this bug on, the Vex will not attack and will not try to distance themselves. And after about 20 seconds, return to the vicinity of the Evoker. When the attack target dies, the Vex is immobilized for 20 seconds.

https://www.youtube.com/watch?v=O7Eep2aPCJA was similar to this.

Steps to Reproduce:

1. Get a evoker spawn egg
2. Dig 8 squares under the ground)
3. Summon an Evoker there.
4. Change it to Survival and have the Evoker summon the Vex.
5. Run away.

Observed Results:
Vex will forever pursue you, attacking you at high speed.

Expected Results:
If the player runs away from the Evoker, the Vex will give up the chase and return to the vicinity of the Evoker.

https://1drv.ms/v/s!AhDMhHfsixt_gWs-5_94eSYfaOv3?e=P2wM8c