Double boss spawn in wetlands map

I was playing a wetlands map node with boss and expedition modifier and as soon as I entered the boss arena the boss (Gorian, the moving earth) regularly spawned as expected. After I reached around 60% of its health another one spawned.

I was notified that the map was completed after I killed the first one (since I kept the bossfight as the last enemy to defeat). Both bosses dropped their loot.

Bug Report ID: #1364672855