As long as this is just happening randomly, I don't feel like we'll be getting anywhere. I've tried to reproduce this on the local server but just can't get anywhere. Instances are working fine for me and I don't know what is done to make this bug occur.
i have a version of why instances bug out(at least at nex), just an assumption tho, because i didnt pay attention to previous kills, but this time when we killed nex, for some reason my character did 1 more attack when nex already died and dropped loot, after that she didnt spawn.
Also, sometimes character will continue attacking while nex has death animation, and this happens when she dies with some little hp left on her hp bar(because of her natural health regen i assume?)
so its more likely to bug out when you're in a group rather if you're soloing( my guess ), will do more nex and pay attention to this detail