When an existing item is referenced as the icon for a new item catalog group, the group cannot be opened, and a dummy group will be created on the existing item.

Demonstration

The following video shows the bug in action:

The final group shown in the category is a custom addition referencing the packed mud item as its icon. It cannot be opened. Additionally, after trying to open the custom group for the first time, the packed mud behaves like a group that also cannot be opened.

Reproduction

This example is taken from the behavior pack in the following world attached for convenience:

This world can be opened (in Minecraft Preview) for inspection or unzipped to access the code associated with such.