When using resource packs that have custom item/entity texts, they don't work on the realm. This is especially prevalent when using functions to execute commands on said items/entities.

This is easily reproducable by creating a simple resource pack and renaming some items, for  example, create a en_US.lang file, and add the following text:

item.snowball.name=Wand
entity.snowball.name=Wand

and observe the /execute working on @e [name=Wand] on an offline map and NOT on a realm. I tried adding all 29 languages with the proper languages.json file to make sure it's not to do with some Realm localization.