I was in Mid doing the mount quest and went to kill venomous spiders in Vanern Swamp. I got one spider to about 50% before I died. I went and did some other quest, then came back to Vanern Swamp to finish with the spiders possibly an hour or more later, and when I got to the same spider, it was still at 50%. After standing in the area near it with it targeted, though, its health finally started to regenerate.
This happened to me with another mob on the same day, though I forget which one it was.
I encountered this again in DF last night. I was farming drinkers and another player was farming molochs next to me. They got one to about 50% health when they died. I clicked on the mob a bit later and found that it was still at 50% health.