The achievement has been frozen, at 388/500 for me, regardless if I add arm layer items to my wardrobe, view that section in my wardrobe, etc.

Yeah, I think it's a known issue that layer achievements don't update correctly on their own.

There was a fix implemented for this though. If you go into your wardrobe and then the "Organize Items" page, you can click on the Layer Achievements section (direct link) and update each layer individually for the achievements you need.

Thank you.