Broken Random Encounter

Was clearing the Mud Burrow on Cruel when I came across some of the poison spitting, burrowing mobs (no idea their name). They were absolutely immune to attack, whether with directly targeted abilities or AoE effects. No damage, untargetable, the whole bit.

They could still attack and do damage.

I noticed when they first aggroed that they were blocking the way to one of the 'prison' encounters (where you have to click the prison a couple of times to release the mob and start the encounter).

After luring the bugged mobs away, I made it to the chamber to discover the issue - the encounter was for a unique variant of one of those mobs, with the adds arranged in a circle around it as expected, except that those adds weren't trapped like they should be. The ones that I had lured away were the ones that had decided to attack me instead of sticking their mouths out of the ground to spit their poison attack.

Once I broke open the prison all the mobs became targetable and I was able to complete the encounter successfully.

This image is taken after I finished the encounter.
https://imgur.com/5AegiVH