@Falz Yeah I see the area drop now.
The reason I thought it was Anga is because the update adding Glass has the notes that:
- Level 76+ Anga's drop table was changed
- Level 81+ Dio Hyunal's drop table was changed
on the JP side. As for what it actually changed? I have no idea.
All I can say is the one Novel i have seen was from anga due to the anga soul on it (it had mark + lucky charm). It was from a summoner who was looking for Conversion Rolls (which still don't drop, thanks sega)

But that said, certain content we don't have on NA yet that gets run a LOT on JP (where it should have dropped if it is in Anga's pool) has not dropped novel, so yeah it's just an area drop as you said. My bad.
It should also be noted Dio Hyunal should be able to drop certain 15* weapons (corrupt partisan/launcher), albeit extremely rarely. And Naverius actually has a Novel drop on JP right now:

Neither of the other ultis have Novels. Although only 2 Ares weapons have dropped in amdu in the last week on JP.
TL;DR: Yeah, Novel seems to be area drop, not Anga specific. What JP changed about Anga's drop pool, I have no idea. Dio Hyunal should be able to drop 15* but it's so obscenely rare it's not even worth trying for imo (especially when partisan hunter has Jild 14* now which is absolutely incredible, unlimited focus works)