@Lodbrot
When you said you received the message that you got them, are you referring to the little pop up in BF1, or the message in BFV?
In BF1 you were supposed to 'claim' the items in the R2BFV event after completing the assignments.
You could have unlocked the Arras stuff in your BFV Armory alongside other unlocks and just didn't notice - try going to your weapon menu amd click on the 'customize' button and see if some weapons have them.
Iirc, it was the Gewehr m.95, Lewis, and the 1906 Selbstlader that got them? *** keep in mind the weapons won't be fully decked out in Arras, only 3 or so pieces can be customized, so look at all the parts to make sure.
If all that doesn't work I recommend you speak with an EA advisor so they can hopefully assist you better