Its a well known bug and occurs with all kinds of unlocks (weapons, dog tags, modules, etc) and progress (challenges, EXP, etc). Its because the game only saves any changes youve made when you LEAVE a ranked server. Its best to make any changes to your classes (including unlocking items) while being in a ranked server pre-game lobby.
You wont lose out on attachments in the long run as the rollback gives you your unlock token back.
... or not. As for me I'm level 48 at the moment and I'm missing 8 more mystery dogtags meaning even if I get the 2 more for level 49 and 50 I'm going to miss 6 more (and btw I'm lev12 on the modules ofc) + the upper half of my support bonus assassment is locked still which shouldn't be + ... but yeah you should do what Hedgehog8 said cause it usually solves the problem but unfortunately there's no guarantee that you won't lose out on attachments/unlocks/dogtags/stats unlike he or she said.
