[PC] Battlefield 1, They Shall not pass. Assignments stat resets to zero!!

Same here!

"Do in one round" assignments reset if you do not unlock the weapon in that round. So you should do the other assignment first, then the "do in one round" assigment to unlock the weapon.

Same thing happened to me on ps4 

I've had the same issue on Xbox One.

It's ridiculous if you need to complete the other unlock criteria first that the game wouldn't say that anywhere. I spent a lot of time trying to get 15 kills in a round with both the M97 Trench Gun Hunter and M1909 Benet-Mercie Telescopic and both have reset to zero. I don't like either of those guns and don't want to keep playing with them, so the weapon assignments ought to be more clear if they need to happen in a specific order. 

I have a similar problem. My asignments are not completing at all! I can make any kills with any weapon required but it doesnt count ☹️

As @CptSkyTR mentioned, you need to do the other part of the assignment before 'In a round' task. It applies for all assignments that have it. According to my information this is not an intentional design, although I have no info regarding if/when it will be sorted out.

Same here. I did the achievement 3 times and the stats keep resetting! So annoying!