I noticed on two different maps, it was Crane and Bifrost, that I had a warlord and it would die for no reason. I brushed it off the first time as just some kind of bug that just happens but the second time I noticed it happening again and I got an idea.
Both times that my warlord died was at the end of a wave. And it was during a time when it took forever to finish the wave. I'm guessing that when it takes too long to finish a wave, the game will automatically kill of the last monster (or the rest of the monsters) and that would include YOUR summoned monster.
I would recommend summoning monsters on maps that are small that way we aren't looking for that last critter and then the game kills it off for us.
My character skin -
In the standard Invasion code there must be a line that tells the game to panic if the last monster isnt found in a certain time and when the panic happens monsters both friendly and questionable all die.