The absorbation hearts seem to use the heart backing for the half-ghost heart.
Not really noticeable with the default graphics, but with another pack it is.
https://i.imgur.com/cOmkIQ7.png
The absorbation hearts also seem to go under the ghost hearts.

Huh. I'm pretty sure I've made it so absorption renders in the 2nd row. Only regular health should render in the 1st row. I'll look into it.

I have the mod with leveluphp, which might be what's causing it.

No that shouldn't be the problem. I've just uploaded the version that should fix this. Absorption should never be rendered in the same row as health. If I tried to get around it and made it render above the ghost hearts it could get messy.