No xp loss on mob death. No recent PvP activity:

Autosaving.
You shiver and suffer.
Your poison wounds you.
You have been KILLED!!

You have died, but the Carrion Fields are not finished with you yet!
Your soul is returned to the realms in the form of a ghost.
You will return to your corporeal state in a short while.
Be careful, for even a ghost should fear some dangers!

PROTECTED civilized <0%hp 0%m 100%mv 3215tnl (63.05%) 11 PM>

First incapacitated by poison, then dead by poison. Phase spider was the culprit.

Since the actual mob isn't the thing killing you, you aren't being penalized from it.