Also when you're joining a public match, and they've already opened the mana chest during the build phase. This is really annoying, because it can also get you kicked, because the rest of the squad could think you're trolling. Until this bug is fixed, I say just join public matches during the battle phase. (PS4)

This also seems to be happening if you are joining a new "match" you have not been in and are the last person to load in.

It even is enough, if you just press G in Build-Phase in any scenario, and leave before the others press G. Then the wave will never start; even if u rejoin.
(On the other side it seems to be a good method to ruin games of people, that pissed u in any way xD) ... 
Worked 100% when i did that. Don't ask, why i did this more then just some times...