Yeah, pretty sure it's a bug. I saw this with Bionic Joints. Upgraded them all the way from Tier 1 to Tier 5 over the course of a play-through and they ended up Tier 5 with 120 armor or something like that. When I un-equipped them at a Ripperdoc they immediately reverted to 85 armor, which is what they are supposed to be at that Tier. I wasn't paying close enough attention to note if my overall armor score changed by that amount or if it was just a display bug on that one piece.