When you set your game to peaceful difficulty in the middle of an illager raid, then any evokers that are present in the raid will despawn instantly as well, even though evokers still exist on peaceful difficulty. Vindicators spawned in a raid don't do the same thing. This only effects evokers spawned in the raid, evokers spawned through other means aren't affected. 

How to replicate:

-Start a raid with your difficulty set to Normal or Hard. 

-Complete the first four waves. Having repeating command blocks to instantly take out all the other raiders would really help speed things up.

-On the fifth wave, find the evoker that spawned, then set your game's difficulty to peaceful.

Expectation:

The raid ends abruptly, but the evoker remains just like the vindicators would, as evokers still exist on peaceful difficulty as passive mobs.

Reality:

The raid ends and the evoker despawns, even though evokers aren't supposed to despawn on peaceful.