I started out with 1/4 possible TA dungeon paths completed (I had only ever done the story mode). Did the Aetherpath, and afterwards the counter was at 3/4. (Then I did up/up and got 4/4.)
I never stepped foot in forward/forward or forward/up (when it was available), so clearly my counter should not be at 4/4.
The same happened to two friends of mine who I ran the dungeon with and who hadn’t done any explorable paths yet either.
I assume that the Aetherpath counts BOTH for forward/up and additionally for itself.