Cannot complete delve encounter. Invincible rhoa with no health bar.

Bug report reference number: 1,397,802,107

I completed a "Nesting Grounds" delve node (the one with 4 bone piles that spawn rhoas) and the chest cannot be opened at the end, nor can I select the delve cart to leave. There is a bone rhoa running around which is invincible and immune to any effects.