Localization systems can use multiple localization keys using %. For example, an item could have the following display name value:

%bug_test.item.muffin.name\n§7%bug_test.fragment.topping§r

This would display the localization value of bug_test.item.muffin.name on one line followed by the localization value of bug_test.fragment.topping in gray text on the next.

Unfortunately, this feature does not work consistently across item name presentation contexts. Specifically, this feature is bugged in UI hovers when not using the display name component to handle key lookups.

Demonstration

The attached images demonstrate this issue, which occurs both for blocks and items. Note that there is no issue for blocks and items using the display name component to reference localization keys. Also note that the hotbar text never has any issue; this problem only occurs in hovers.

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.