Same thing happening over here, im thinking probably whoever maintains the flatpak needs to update some stuff

Probably related to this last commit? https://github.com/flathub/com.jagex.RuneScape/commit/ff83d6c531d5f3ca827be85ace2d0090de365d87 Which seems to have changed the sizes and sha256 values

edit: Ok i think i figured it out looking at some issues on their github, basically whats happening is the .deb files get changed without a version increase, and because the flathub pulls from that it causes a mismatch in the files. I think the commit that i linked is there to fix it but might take some time for it to update on the flathub servers