The owner/summoner of a vex is not stored to NBT, which means it is discarded when leaving and rejoining a world.

How to reproduce (22w04a)

1. Summon an evoker
2. Go to survival mode
3. Run the following command to confirm that the final say command runs as the evoker

   /execute if entity @e[type=vex] on origin run say hi

4. Leave and rejoin the world
5. Run the same command from 3. again and notice that it can't find the origin of the vexes.