Been there, done that title
Titles are rewards for achievements, and achievements are account wide. You can’t “lose” a title or an achievement for any reason (eg collections: you keep the collections and every progress on it, even when you delete the collection items)
You lose the star that was on that char’s nameplate that showed it had done the 100% map. The title is account wide and you keep that.
Be careful what you ask for
ANet may give it to you.
ANet may give it to you.