I'm seeing health in some instances (an armored wither skeleton spawned by Doppelganger) reported as, like, 4.7 or 1.35 which I don't quite understand, as I was under the impression that "hearts" are two health points and you can't have fractions of HP.

What's going on?

In Minecraft, health is actually internally a floating-point number (a decimal), not integer values. So yes, this actually is possible, and not a bug.

( ... @totemo ?)