Not much point in posting here, but runic mobs won't die

I've killed a couple of empowered elderslayers now, not a benchmark but they are pretty tough imo and should in theory be one of the higher HP pool mobs someone at my point in the game would fight, especially with the wisps crapping their numbers all over the place.

Just went on an expedition though, I can't kill a couple runic mobs. I wiped everything else, I killed the heroslayer boss guy, I didn't use any modifiers placing the dynamite that would nerf my main damage output, which is fire. Yet, they won't die. They don't hit hard, I just ran around grabbing my loot, talking with whatever guy about the text I found and ignoring them.

I'm curious though if there is any concept of "working as intended" applied against some of the plethora of absurd numbers behind the mobs in this game? Is this working as intended? Spend 40 minutes killing something, or something that you can't kill at all that is just a rare mob or less? Or the 1 second surprise degens or one-hits from under-leveled trash mobs? Does GGG actually release leagues like this with the desired end-state being invincible random rares? If not, my bad, please fix it. If so... damn. Do you guys read some of the posts in the feedback section and go, "yup, that's what it is supposed to do"?

Runic monsters have some of the strongest modifiers in the game.

You can make them immune to your main attack damage type. These immunity pylons are color-coded.

You can make them take 90% less damage (stack 3 pylons 30% each)
You can make them have a 90% chance to avoid damage from hits (also 3x30)
You can make them have 95% chance to block attack and spell damage (only 2 pylons needed)
Max life as ward combined with max life as ES is also nasty.

Most players don't want to bother with manually settning 7 explosives and take the one giant explosion keystone. If you catch such pylons in the radius you won't be able to kill those runic monsters.

Definitely something fishy going on with expedition monsters, because I had empowered them and killed almost all of them, but 2 or 3 random white runic monsters were taking ZERO damage(and im on a glass-cannon build with insane dps, im talking over 200 mil on my single-target skill gem). Then I switched to attacking other mobs, waited for those immortal monsters to move around a bit and suddenly they died super fast? Seems like a weird bug but it only happened to me once, but I can definitely confirm it did happen.