"They can't see me! I can't see them!"
"That's because you're facing the rock..."

Anyway, being hit by shit that hasn't even loaded on your screen due to massive swarms of PCs/mobs has been a thing since FFXI's Besieged. Tough, but you gotta deal. There's no magical fix for this aside from putting stricter limitations on how many PCs can be in the zone (and I *still* see hunt mobs occasionally not loading even today if enough people are there).