@Ninja Dankinate I believe I may be able to fix that. If you go to Minecraft settings (not the realm or world settings), scroll down to 'Storage' and find 'Cached Data'. You can then delete everything in that section and it should be fine (cached data is just the resource paks applied to servers and realms. It stops you from having to download the packs every time).
If you can't see cached data, you'll probably have to find it in your file app and delete it there (I don't know why it doesn't always show up).
Affects 1.17.10
@Ninja Dankinate, interesting. That makes it seem like another pack with the default textures is on the realm. Is it possible that the default texture is still added? I think that you can remove it from a realm. Other than that, maybe go through the other packs and see if those might be the problem.