Players playing starlingrad campaign have to get xp to reach those hiding levels and get nothing as reward.
It is not a bug, the stalingrad pass has not been available for purchase now
If you do not have the pass you will level up to the part that the game tells you, also if you have any other campaign played maybe you can send your weapons from there to stalingrad (mkb42, gewer41, etc…)
I finished the lv14, the next lv displays is lv16 and I found that the xp is 0 for lv16 after I played server games and when I finished a game i see I’m filling xp for the hidden lv15, I just want to make sure that this confusing behaviour is designed by your PM team.
Yep, you should spend your time and xp for the levels that not even existing anymore…
Working as intended, comrade! Move along, nothing to see!!
Now that you put it like that, it sounds very silly indeed 
It used to have pay to unlock levels, but they removed that due merge announcement and you no longer can spend money on it to get those contents, yeah $talingrad is P2W in case you didn’t notice it
What?
Stalingrad engineers with mkb42s all the way