Behavior packs apply to realms but don't work once you load the realm. The behavior pack just says it is missing when you go to exit the realm to check on the issue. Even when the behavior pack doesn't have any bugs or errors with it. It works on local worlds fine and I'm sure it is not an issue with pack because every single behavior pack I've tried has the issue. I am not sure if there is a bug with the resource pack though, all I know is that addons don't work at all on realms right now. It is probably not to do with the resource packs because my custom resource pack works just fine in my realm with no issues whatsoever. The only thing from the behavior packs that work are the entities which appear when you enter the creative mode item list. But the entities are invisible when you spawn them.