Ok, in short, I repeat all steps:
1. I bought a Mask in Ados. Nice look ! :)
2. Soon in 30 mins I died and appeared in the Temple without any mask (in my standart look).
3. I continued to play.
4. In several hours (about 2 or 3) i've got the message: "Your costume is weared off" (Somthing like it...)
Well, I think I clearly described this little problem... My ingame name: testik