I made a video showing this problem in action https://www.youtube.com/watch?v=qAnWerdKhfw
The most simple solution would be if players are unslotted when they abort a mission. This also happens in respawn types "BIRD", but there it is not a problem since you spectate anyways when joining in progress. In respawn type "GROUP" (and likely "SIDE" aswell) players can cheat themselves back into a no respawn game.