Miniature Collector Set 1
I don’t think this is a bug, it is probably because of an old mistake some dev made lol. When they added Priest of Balthazar mini they accidentally put it in Set I, and when they noticed it was too late and/or they didn’t care. This meant that you could get the pre-wardrobe set I achieve with only 53 of the set I minis if you had the Priest too. The new achievement correctly requires all of set I though. I’m in the same boat as you right now…
Here is an old screenshot of when I had 54/54. Impossible to track with the new arrangement!
http://www.thehotrepublic.com/wp-content/uploads/2012/12/Guild-Wars-2-Mini-Collection-2012-12-19.jpg
I have Balthazar mini , and Set 1 54/54 in Wardrode Storage