Im not sure if this has been brought to your attention yet, 343, but wins are no longer registering, and cant have been for a long time.
When I go to halo 5 in my xbox game hub (the place where it has your game stats and ranks you against your friends) it says I have 24 wins despite having won hundereds of games by now, winning does not increase this number either.
I started playing halo 5 on this account this january, and won atleast 100 games all up back then so this bug has been present for a while.
If it is happening for me it is certainly happening for many players… i cant help but think this bug is tied to the warzone achievement not unlocking.