Update: So It looks like ‘Quick Play’ is working for progression, I got +9 kills added from ‘Quick Play’ for 9/50 progression, but ‘FFA’ is still busted for me and not counting toward my weekly challenge progression.
I played two ‘FFA’ matches, in one I got +25 kills. At the end of the match, it said my score was 0 with 0/50 progression for my final weekly challenge. I’ve restarted the game and it is still not registering. I even tried a ‘Bot Bootcamp’ match (since the challenge doesn’t say in ‘PvP’), that gave me 0 score as well with 0/50 progression. No matches are counting toward my weekly challenge progression (except ‘Quick Play’ at this point).
It is entirely possible that ‘343’ originally coded the weekly challenges to only work with the original game modes which were present during the initial multiplayer release, and not ‘FFA/Tactical Slayer’, ect. This could potentially be why some players are experiencing these issues, while others are not.