Invincible mobs seem to prevent strongbox opening

I opened a Strange barrel that was near some other strongboxes. I went to open one of the regular ones and it didn't open after I killed all the mobs with the strongbox indicator. I noticed later that there were 5 untargetable and unkillable spiders nearby. They aren't dangerous and it's just one strongbox, but it's strange behavior.


I don't remember the timing of when I clicked the second strongbox compared to the Strange barrel. It may have been before the last wave or after, but it was definitely after I started it. I do remember i was spam clicking the barrel between waves. I killed all the other mobs in the map to make sure I didn't miss the strongbox monster.

Here's a video of the bugged bugs https://youtu.be/czGT5hzLBxk

forgot to include the bug report ID: Bug Report ID: #4,167,768,913