UtilityBelt/src/main/resources/data/utilitybelt/tags/items/allowed_in_utility_belt_belt.json ends with _belt_belt.json, but it should be _belt.json. I noticed while setting up a datapack to add some items to the belt. When making a datapack, the file actually has to be named allowed_in_utility_belt.json, adding the extra _belt makes it not work. The issue is simply that the file packaged inside the mod is named incorrectly. If it were not empty, this would probably make it not function, but since the file doesn't actually add anything to the tag anyways this has no effect in-game.

Thanks for reporting