I mean often the player is to blame, almost 100% of my deaths were due to my own negligence. Then there can be bugs or just a huge amount of bad luck. I think people are surprised by your story because most never had to fight fuelings and trolls in the mountains, let alone close to a Morder altar since those tend to be pretty high up away from biome borders.

Most of the times you can avoid pulling additional enemies by going slowly and either avoiding or killing them before you move further. Unless this is some new kind of bug I have never heard of, it sounds to me like your case was a combination of bad luck and wanting to rush things. Which has gotten me killed a couple of times, too. That's why like someone said before here it always helps to have multiple portals on each island in case things go south, even if it takes extra time and materials.

If it was a bug, it provably should be moved to the bug report forum.