The hardcore-signifying markings on absorption hearts in Hardcore mode are extremely light, and almost impossible to notice without really looking hard. they also appear to be asymmetric, as the marking only appears on the left half.
A fixed icons.png that makes the markings more akin to usual hearts is attached. (Although I'd really suggest just splitting up icons.png into its constituent sprites instead.)
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As for the hearts not centering, I think thats normal, I don't think hearts have ever been entered in normal gameplay and I believe that hotter icons on the left side of your screen will always start at the left and vice versa.
This is related to MC-18880.
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I can confirm for 1.16.4 (though this might be a feature request)