Quests with multiple IDs cannot always be restarted
Jonado on Discord:
I found one more bug, and this is definitely on the engine side (checked in an OpenMW version from about four weeks ago). If having two quests with the same name (let's call these entities "subquests" henceforth), they essentially work as the same quest, also regarding journal entries that mark the quest as "Finished" etc. Now, if finishing the quest from one subquest, a "Restart" entry from another subquest does not currently reactivate the quest in the journal - for that the entries need to be part of the same subquest. I hope this will get fixed soon.
As noted by Scrawl, vanilla stores quest state by name whereas OpenMW keeps a state for each ID and then decides the overall quest state by iterating over all quests and if one of them is finished, the quest with that name is finished.