Hello!
I was trying to get the Instant Karma achievement while playing the Necromancer and discovered that cursing the Forgotten skeletons causes their deaths to not count.
I was playing solo on a Seasonal character. I did not have either achievement (there is one for non-Seasonal as well). The curse in question was Decrepify with Dizzying Curse rune.
I am positive I was not killing them with Thorns, Area Damage, etc.
This should be easily reproducible. I made several new games over ~3 hours, thinking I was doing something wrong, and it happened in all of them.