Completing a Magic Woods, while it rightly adds +1 to the Mwoods completion stat, it doesn’t count as a ‘dungeon complete’.
See screenshot where I’ve completed 1 Mwoods and 2 Pcaves, but only have 2 dungeons completed instead of 3:
Edit:
And might be worth saying that this is not merely a bug on the live stats display screen, the data itself isn’t recording correctly on the character stats. It also shows wrong when a character dies on its fame/death stats screen, undercounting total Dungeons Completed.
Edit2:
This bug also definitely affects Cnidarian Reef completions, possibly Secluded Thicket, and so maybe others of Deca’s dungeons too.