Unfortunately, I believe the Achievement mod is incompatible since the armour DLC was released a couple of game updates ago; there have been several reports of it no longer working.
Was the counter going up in your current playthrough, or could that have been from a previous one, if applicable? Or did you take a break before the Jun 14th update and recently return to the game?
When I tested the achievement some time ago, the counter usually didn't go up until I exited Steam and restarted (in once case it went up after a few minutes, while I was doing something else in-game), though the achievement itself unlocked immediately. When there was a problem with the achievement in the Definitive Edition, a couple of people that played the game in the original release reloaded a Nameless Isle save from their earlier playthrough and got the achievement in the Classic version.
EDIT: see the reply below for (possibly) an easier fix.
Try creating a new profile, than copy a save from the start of the Nameless Isle there. If you load the save without any mods enabled, hopefully that will let you get the achievement, if it is just a case of the achievement mod no longer working. If there are later achievements you still need to get, you may want to try disabling mods in your existing profile, and continuing without them.
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
There is a
D:OS 2 save editor which has a function to remove mods from saves, which might help (I have not tried it), and you can add some boosts to a piece of equipment your characters can use temporarily to speed up the required fights (having already gone though some of them twice), and if possible lower the difficulty temporarily, etc.
