Weird boss interactions in Heliophage, might be a bug

So a couple of times I’ve played through the Heliophage with groups, I had a particular set of circumstances come up with a weird result. I’m wondering if I’m the only one noticing this, or if I’m just misinterpreting a known glitch.

Now, just for the record, I’ve only seen this on Advanced Heliophage and I don’t know if that’s a factor. But in the ‘fight bosses from the other levels’ phase of the stage, a couple of times I’ve fought Jailer Hylis with one wave of bosses and Foreman Grall with the following wave. (Both times it was the first two waves, I don’t know if that’s a factor or not.)

So normally when you fight Grall, multiple times he puts up that shield, you have to fight the phantoms and Varelsi enemies, and so forth. But when I’ve gotten Grall following Hylis, he almost never puts up his shield and Hylis Guards keep spawning. I don’t know what the trigger is (maybe distance from the Guards, maybe it’s a time thing) because it’s not like this is an easy thing for one person to test, but eventually he puts the shield up and the Guards will stop spawning.

Now, it’s a lot easier to fight Grall that way (as long as you can deal with the fact that the Guards are a pain in the neck) since he isn’t putting up the shield, but I’m just wondering if anyone else has noticed this?