When you Import a world to a realm and it has experimental settings on, it will turn off every single one but caves and cliffs if it was on using the world file, this can be shown in realms using addons/mods the most. because all of the items just disappear if they were using the new format and most of the blocks become the Update Info Block if they were using the new format versions. and when you go into your modded realm you will see that the mods are installed correctly.