I got the same problem, i can play normally but my chat disappears after the briefing, when i start playing i can't see it anymore, while others can see what i type, it's pretty boring alt tab everytime cause of steam messing up ArmA 3 for the ingame chat of steam.
I got the 0.77.1 I have this problem since months but, i rarely play MP cause of every server is Wasteland, so i don't really need it, but i noticed in editor if i make a script about chatting it won't show up.
P.S. Sorry for the offtopic, but i can't post new threads.