Seems to be behaving moreso like a bug than anything, but I'll treat it as feedback for the moment.

This seems to have been a minor issue for some time now, nothing really game-breaking or serious PvM-wise. But with the new Ancient Caves and these new mobs, I've noticed quite quickly how these "Single-Combat" areas don't necessarily behave like a single-combat area most of the time.
Anyone who has ever been to Nechryaels in the Smoke Dungeon will probably be familiar with how they will occasionally pile onto you for some unknown reason, despite you being in a single-combat zone and already in combat with another Nechryael. This occasionally results in your current Necryael becoming untouchable for a brief moment which tends to get frustrating for the umpteenth time it happens. From my experience, this also occasionally happens with the Metal Dragons & Brutal Green Dragons when 2 attack you at once, but this issue quickly fixes itself.

These new mobs are no exception, only being a much higher combat level (and much more relentless with their potential damage output). While this doesn't happen with Melee from what I've noticed, it seems to happen fairly often with Magic. I have yet to try out Ranged, from what I recall I do remember splashing occasionally making the current mob you're fighting in the past unaggressive, and I have only so far tested this out on Ancient Gladiators, so the only issue might be that I am using Magic. But regardless, when I accidentally do not deal damage for a short while, then have 2 or 3 Ancient Gladiators charging at me (Or smacking me away with the other attack, making it incredibly hard to attack the one I currently am fighting), it makes one of the few viable ways to get a decent killcount per trip very unappealing. Adding to this the incredible amount of damage they have the possibility of hitting me for (I've already been hit up to around 40-50 from this, nothing too insane yet), it puts me at a pretty high risk of being demolished by mobs I am not even currently fighting.

How certain aggressive mobs and single-combat areas work together just tends to feel a bit wonky from time to time. I feel that the new mobs are fairly balanced for their difficulty, but become much more of a nuisance when they attack you out of nowhere while you're in the middle of a fight with another mob. Would it be a possibility to look into simply making them unaggressive so this doesn't remain an issue?

The problem, particularly at Ancient Gladiators, seems to be that while you are in combat with one of them, if it does not inflict any damage upon you for a certain period of time (due to being frozen by barrage - it can't reach you) then another NPC is able to hit you atleast once. Once you're hit, occasionally the monster you were originally fighting losses agro on you and you cannot attack it as you are under attack by other NPC.
The thing that makes it particularly annoying at Gladiators is the fact you are sent flying by one of their attacks, it's just a real pain in the ass.