Glitch with mobs' health regeneration?

Damewort - Sanctuary

Today noticed that mobs' health doesn't regenerate anymore. It should be so or is it just me?

Explaining:
If you attack a mob and later stop killing him before he dies, then after it loses aggro it hp regenerates till it is full, right? Well... not anymore... was forced to run away as too many mobs attacked at the same time. The mob I was attacking was still selected and even after stopping to chase after me and returning to its place, mobs hp was still at 10%. Thought it is just a graphic problem, so went back and really took it down with one blow.
Tested it on another mob - same result.