A lot of applications on Flathub use the KDE 5.15 runtime to avoid having to build their own version of Qt 5.x. Unfortunately, the 5.15 runtime was deemed obsolete: https://github.com/KDE/flatpak-kde-runtime/commit/1d5ed66ae8338d93d48a9485f103b9e0f5fe9b64 without a viable Qt5 replacement for all those applications that still use Qt5. I ran into this for FlashPrint (proprietary): https://github.com/flathub/com.flashforge.FlashPrint/issues/22 and GBE+ Either adding the Qt5 to the runtime (although the 5.15 LTS support cycle is due to end in 2023), or to the Flathub shared-modules (https://github.com/flathub/shared-modules/) would be good work-arounds.
Use 5.15-21.08