I don't know it is a bug or not. The guide say it can attack entities. So I try to put an Armor Stand on the Life Injector, and it works. I think in this way you can get infinity health without killing mobs. But I don't know whether it is what you want. So I submit this issue to ask it.

Definitely not intended...oops

Armor stands (and other things) should now take damage in the new version