It's still not confirmed, but it's looking likely that this quest may be the cause of the new bug that desyncs your character's quest states. See the discussion here:
https://forums.elderscrollsonline.com/en/discussion/636606/quest-status-bug
If you have the bug, you'll notice that the zone guides in various zones will show a lot of things as uncompleted, even though you may have done the quests, and it may even leave certain quest hubs in unexpected states (e.g. the weather in Vvardenfell may be stuck in the state where the volcano's erupting). In general, the quests the zone guide says are completed tend to be in odd orders, e.g. the last quests finished but the first ones unfinished. The bug has also caused zones that have not been completed to look like they have been (most affected players are now showing 8/9 story quests completed in Necrom despite not touching the story quest. This does not allow you to redo quests, but it does leave the zones in this mismatched state.
While it seems mostly visual, it will end up locking you out of quest-locked zones, so you will end up blocked from finishing certain Chapter questlines that have quest-locked areas. Several people have also seen that Tales of Tribute queueing is also not working as the game assumes you have not done the tutorial, but it won't let you redo any quests.
I'd suggest that until this is investigated, it may be helpful to drop the "Kill 20 Arcanists" quest from your log for the time being.
Edited by ZOS_Icy on 23 July 2023 16:56