Exactly what it says on the tin, when passing through dimensions or logging and relogging zombie plains villagers may change professions at random; with this change continuing until the plains zombie villager reverts to the profession it spawned with.
To recreate: Naturally spawn a zombie plains villager, name tag or boat it for despawn protection, then log out and relog in front of the zombie.
Reproduced in 26.1-snapshot-6
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This does not happen every time.
The NBT data sometimes mismatches the texture and upon unloading and reloading it changes to the profession listed in the NBT data.
Upon relogging: