note: as icons.png has finally been split as of 23w31a, this ticket will need redone.

The bug

In 20w48a and prior, by modifying icons.png, it was possible for absorption hearts to be given unique textures for when under the poison or wither status effects. The regions corresponding to these hearts was left blank in vanilla (hence MC-18880), however they could be filled in with a custom icons.png (see attached) and would work as expected. However, as of 20w49a, these custom hearts will no longer be displayed, which breaks any resource packs which used this feature previously.

To reproduce, use the attached resource pack, which changes the heart texture spaces accordingly, and have absorption in combination with either wither or poison. Note how the extra hearts gain a unique texture in 20w48a, but not in 20w49a.

To fix, the poisoned and withered absorption hearts could be reassigned to a new spot in icons.png (as to avoid conflict with frozen hearts and such) and given textures either duplicating normal absorption hearts (as to keep the current visuals while still allowing resource packs to change these) or new unique ones, although a better option would be to just split each heart into its own file as icons.png is already enough of a mess and was likely the reason this functionality went unheard of to the developers.