Text presentation systems can make use of emoticons registered to the font/emoticons.json file in a resource pack. An example emoticon fragment usable in text is :shank:, which displays a hunger shank as an icon. This icon works correctly in chat and hovers. Unfortunately, it does not display the icon in hotbar text when switching between items.

Demonstration

The attached images demonstrate this issue, which occurs both for blocks and items regardless of whether each uses the minecraft:display_name component or not. Note that the hover text never has any issue; this problem only occurs in hotbar text.

Reproduction

The demo screenshots and code are taken from the behavior pack in the attached world for convenience. This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such.