In my club, I got the "Get 10 5ks on World" mission. I went to Singleplayer (hover dropdown) -> World (the one with the new interface and the friend leaderboard) and got 6+ 5ks. Then, I wanted to take a break so I checked it... and it didn't count!!
I then clicked Singleplayer -> Maps -> World instead, and it counted. This doesn't seem like the intended behaviour.
Chromium 153.0.8010.52 (Official Build) Linux