If a world is closed and reopened while a fang attack is underway, the fangs become able to damage illagers.

Steps to reproduce:

1. Build two pens to hold evokers, as shown in the attached image.
2. Stand in line with the pens and run the command

   /summon minecraft:villager ~ ~ ~ {NoAI:1,Invulnerable:1}

3. Turn on hitboxes with F3 + B, so you can tell when a fang attack is just beginning.
4. When the evoker furthest from the villager begins their attack, just as the hitboxes appear, hit Esc (or the equivalent on your computer) and Save and Quit. Rapidly tapping Esc in the moments leading up to an attack may help.
5. Reopen the world and watch the other evoker get damaged by the attack.

I personally set up a repeating command block to teleport vexes into the void with

/tp @e[type=vex] ~ -1000 ~

but that's optional.

Player-spawned evocation fangs can damage illagers (not sure if this is intentional), and there's no data tag that records the difference. I'm pretty sure that that is the cause of the bug.