While soloing City of Ash I with Bastian, he became unsummoned midway through the fight with the final boss. I managed to get far enough away from the boss to cause the "joining encounter in progress" to trigger, and after doing that a few times I managed to reset the boss and summon Bastian again before aggroing the boss.
Then it happened a second time, even though I was careful to keep my Quickslots wheel on something else (a stack of Weapon Power potions). The second time, I was not able to reset the boss and summon Bastian again, so I shut off my computer, powered back up again, and logged back in within enough time to get back into the dungeon without resetting it.
When I summoned Bastian the third time, I removed him from the Quickslots to ensure that he wasn't being accidentally unsummoned by me. And partway through the fight, he disappeared again (meaning his info no longer showed in the "grouped players" section of the screen), but then he reappeared after a second.
As best as I could tell, the moment he disappeared coincided with me having to jump into one of the ditches of water that criss-cross the area of the final fight. When I jumped or climbed back out of the water, Bastian reappeared.
I've fought mudcrabs more fearsome than me!