This has been a problem since the beginning of the game's release. I even brought it to SH Game's attention a couple months back, but no answer. It is definitely a bug, and either they forgot to add Advanced Rifling to the PDW, or they added the challenge completion for Advanced Rifling on accident.
This happens when you do unlock all from a tool via rpc. Same thing happens with exo equipment etc.
That doesn't apply to me though so I guess the bug theory still stands