im currently using the latest release (1.1.0)

With my testing horse hearts do appear in creative but not survival, and while in creative the hearts do update properly (in case this meant anything)

Note: for testing purposes i have no other mods installed, only Exordium and Fabric API

Probably the health bar didn't notice that it changed, so it didn't re-render. Taking damage/changing hunger etc probably fixes it. Adding the missing condition.

Edit: apparently that's not the issue, which is really weird.