I think I might’ve figured out the reason! I had a save file from before I evolved my starter Teddiursa into Ursaring, so I did some testing using that.
As you know, in the original game, starter Pokémon can only evolve after beating Darkrai and unlock access to Marine Resort. But in this version, it seems that using the Ascend Stone Mark 1 allows you to evolve them right after the first ending. So naturally, I thought I’d be able to evolve Ursaring into Ursaluna too — but apparently that’s not the case.
Before evolving my starter Teddiursa, I recruited another Ursaring from a dungeon, and that one was able to evolve into Ursaluna just fine. But once I evolved my starter Teddiursa into Ursaring, both my Ursarings (the starter one and the recruited one) became unable to evolve into Ursaluna.
So I guess the game is still restricting starter evolutions past a certain point, even with the Ascend Stone? It’s a bit disappointing, but maybe I just have to wait until after the second ending.
Still, if there’s any other way to evolve it earlier, I’d really appreciate it if you could let me know.
Thanks again for always replying!