I'll start off by saying that I don't know if this is actually supposed to happen or not, but it never happened on Esior's Haven raids, so I'll post it.
Anyways, I participated in last night's Fungus Grove raid. I got to the boss but refreshed the page and saw that the boss was already defeated. I took one turn to see if the page would say the boss didn't exist or not, and it didn't go away. I tried the "Stuck in Battle" button just in case and didn't get any results. So I just waited until today to see if the boss would magically disappear, and he's still there.
Again, I'm still a bit new and haven't really done any raids like this. Pardon me if I'm wrong.

@Palada,

The raids allow you to finish your battles against enemies as long as the zone is still accessible.

Since Fungus Grove is always accessible, you will remain in the battle until you are victorious or defeated.

This is to allow more than 1 person the chance at defeating Event Bosses.

Ah, okay. Thanks for telling me.
I'll try my best to kill him.