icons.png contains slots for poisoned and withered absorption hearts. The vanilla resource pack leaves these slots blank (hence MC-18880), however resource packs are able to take full control of them.

However, as of 20w46a, frozen heart textures have been added, and their flashing damage texture is assigned to the same empty spot poison-absorption is (hence MC-206881). As a result, any resource pack that adds exclusive textures for poisoned absorption hearts cannot add unique textures for damaged flashing frozen hearts.

To fix this, either the flashing frozen heart sprite should be relocated to a different part of icons.png (there are plenty of disused and never-used icons that could be removed to fit these in), or the poisoned/wither absorption hearts should be moved somewhere else instead as to allow resource pack creators to still give poisoned/withered absorption hearts unique textures.

A resource pack (and the contained icons.png) are attached that allow this effect to be seen.