Enemy falls underground

While fighting My'ternes, Mother of Webs I thought I'll trick her and she falls down a cliff and takes some damage, was less than half HP. I'm not sure what happened, I was running around healing myself, and when I looked back I saw no spider and followed the indicator, it was pointing down inside the rock. Spider was dead, no parts were visible outside the rock, but I could slash with knife and skin it.

I had a similar encounter before fighting the same pack near the water. One smaller dead spider fell down to water, like 5-7m deep, after a while another did the same but looked alive. Big one also did that, but managed to reappear on the surface. What I did that time was I jumped/glided over the water to nearby rock on mainland and enemy couldn't find a path to my location OR my follower blocked it, because sometimes they were running around to me. This is really strange behaviour. Seems like they can't climb certain amount of height (both up and down) or something and they get blocked in that area. Same thing with some caves where spiders spawn, they can't climb down that natural stone platform, but when shooting at them they just regenerate HP.