If you were to kill an evoker or illusioner while it's casting a spell, it continues to do that and create particles unless you relog.

To replicate this behavior, use the following command to summon an evoker or illusioner and have them start to cast any spell. Then, kill them while they are still in the process of casting the spell to see the particles continue to generate by them.

/summon evoker ~ ~ ~ {Health:1b,DeathTime:-500s}

/summon illusioner ~ ~ ~ {Health:1b,DeathTime:-500s}

You would expect the particles to stop generating once the evoker or illusioner dies, but they will continue to do this until the timer expires or you relog.

*Note, I know that the illusioner isn't a mob that isn't being worked on anymore via report MC-48071, but I am grouping it in with the evoker because it has the exact same behavior as the evoker in this regard.