Half of map boss goes for a walk outside arena (unfinishable)

In Derelict Mansion, there are two bosses. One of them, Varloch, was kind of floating outside of his usual position at the start, but I didn't pay much attention. After killing Avelyne, I noticed nobody's attacking me. As can hopefully be seen on the screenshot, Varloch seems to dwell outside the arena. Since i only had one portal, I do not know if quitting would make it possible to re-enter and fight the boss. When I quit and do not go directly to the hideout (instead appear back in the boss arena), the situation stays the same.