I have this issue. Summary: I cannot change the settings of a resource pack on my realm. I am able to upload the pack, but whenever I go to edit my world on the realm and change the setting within the pack, after closing (presumably saving) and opening it again, it is reverted to the default setting.
Makes most packs pretty unusable, hoping it gets resolved soon.
Not using the same resource pack as OP, but encountering basically the same issue (and I have too in the past). In my case, the subpack sliders does not even show when opening the resource pack within the realm. Downloading de realm, changing the world_resource_packs.json and replacing the realm again does not resolve this, the resource pack goes back to its original setting. Everywhere else, the resourcepack works fine, this is deifinitely a realm issue, which needs to be addressed.