That's just it. Since your asking it NOT to split beyond that everything is always going to the same drive.
meaning if you tell it to store Movies on Disk 1 then it'll never go beyond disk 1.
I used to be really bad at making sure all my seasons of a show are in the same Tv Folder. Now I'm just making sure all of the files from say Lucifer Season 1 remain together. I don't care about Season 2 or Season 3 as long as ALL episodes from a Season stay together. When you drill down to TV or Movie shares it displays them all together, because unraid is doing all the magic in the back end. Plus Plex, Emby or whatever you use don't care either.