Invulnerable and Untargetable Monsters (likely from strongbox)

Encountered some invulnerable mobs in a map. I'm pretty sure these spawned from the strange barrel strongbox. I have encountered similar scenarios recently but in those cases, the mobs were actually invisible as well so I was unable to get any clear evidence. I have recorded a video of the interaction.

https://youtu.be/O-vCWVikA0Q

Bug Report ID: #2,807,452,514