Same here. I opened Battlefield 1 on my Xbox One S and after I finished the campaign (got the game only since a couple of days), as I received the new gun as a reward I went to see what kind of gun it was in the Customize Menu.
The Customize menu takes quite a while to open (about 45-60 secs) and everything was like if I never played while, yesterday, I remember I reached, at least, Rank 4 with my medic yesterday.
Not only that, but I also couldn't get access to ANY of the stuff I unlocked through packs. (I used the 30-something packs that came with the whole season/DLC things.) Now, everything seems to be locked. Even the gun I supposedly have unlocked by completing the campaign.
As I read around, I have tried to join a game and my stuff seems to be available from within a game. After, if I return to the main menu and then to the customize menu, it's back to Rank 0 with nothing unlocked.
(So, I can access what I have unlocked previously while in a game, but can't access it while in the customize menu. I can't unlock new stuff either since my Warbonds count is also reset to 0 in the customize menu.
I still haven't tried to play a full match yet. (Don't have the time for now.) Will check it out tomorrow.
My XBox One ID is donoghu1987