I was in a situation where Archon Boreal was using different means of immunity, almost back-to-back.
Different than usual:
-He was using his scream shield very frequently, and I couldn't interrupt it with any means.
-I was familiar with the part where he would summon adds which need to be defeated before the fight could continue, BUT that stage repeatedly occurred (at least 4 times) while the boss was at 50% health. I could swear they were previously only summoned when the boss loses 25% health.
I've fought Archon Boreal before, and have never seen these issues. I wouldn't be shocked if it's due to Overguard changes, like other folks suggested.