When trying to view a replay, a custom file chooser dialog opens to select a folder. As far as I can tell, there is no way to view hidden files, so if you have installed the OpenRSC Flatpak (as I have) you cannot navigate to the folder and must manually paste the location of a replay file into the "Folder name" field.
Ideally a native file chooser dialog should be displayed via xdg-desktop-portal.