Forgive me if this has been addressed. I'm on mobile and haven't had much luck with the search feature with the quest title. Also forgive me if this isn't the correct place; I'm not sure if this constitutes a bug. If not, it's more of a suggestion.

For "Onwards, to Masteria!", is the quest SUPPOSED to remain under the Maple Mailbox Available Quests menu once the saga has been completed? Or am I missing something as far as the quest line itself?

I could see it being kept in the event that one wants to revisit the Masteria story. But it's really annoying to not be able to clear out this list because of it. If it's supposed to be there, consider this a suggestion: perhaps simply move it to the actual Quest menu (i.e. the hotkey Q menu) under Available, but remove it from the Maple Mailbox, as that's quite an aggravating place for it to be. At least on the Quest menu, there are plenty that remain after completion, so emptying it out isn't much of an option, anyway.