Impossible to install flatpakref files double-clicking the file in Dolphin on neon dev unstable. I can do it on Arch using Discover 5.11.
Possibly an old mimetype database on KDE Neon? org.kde.discover-flatpak.desktop is responsible for this.
Tried to install latest libreoffice through a .flatpakref file - it just opens as a text file. Could be nice if the system knew how to open it. Tried a ton of variations, including plasma-discover --local-filename org.libreoffice.LibreOffice.flatpakref I couldn't make it work... Using KDE Neon user edition
Probably a dupe of bug #389724 I'd say
Fixed on neon dev unstable bionic.