Just for the records, there is a long story in https://github.com/geany/geany/issues/1303 about AppImages and Flatpaks.
I don't think it is likely that we (Geany maintainers) will provide a Flatpak package officially. We don't have so much resources (in terms of time) and nobody of us (as far as I know) uses Geany via Flatpak. And so the motivation to invest time for having a well maintained Flatpak image is rather low.
And all what @elextr said.