I have a handful of mods added to help me test the game and admittedly this is the only time it's happened (and I've been doing a lot of AS fights), but I figure I should toss it on here. I was fighting AS, he went into his "fuck off from the fight" phase but he just never turned around and came back down. He also never despawned, even when he was so far away that scrolling out to max on the map didn't show his icon.
Forcibly despawning him using DragonLens and resummoning worked fine, and I had a normal fight after. I think this was just a one-off thing.
Specifically I was fighting him with 1 Vengeful Sun Staff + the rest Mounted Scanner, using the tiercord post-mech3 loadout. I tend to spam right click to reposition the Mounted Scanner, donno if that has any interaction.