On the xbox version - leaving crashes and disconnects aside - when a random player joins and starts loading in, the bots disappear. On higher difficulties this is a problem because I often run a group of two, and when we turn around, the bots are gone and we are alone for several minutes while they load in. I assume leaving the bots in until those players spawn is an easier solution than having the players load faster or spawn on the group, although those would both be great as well.
The game is a lot of fun, which is why I talked people into buying it, but the online functionality feels dated.