I can confirm this problem with Sway. Selected text can be only pasted within Geany. It doesn't even work with other Wayland native softwares, like Alacritty. A bug is already opened on Alacritty side : https://github.com/alacritty/alacritty/issues/3592
The same bug also applies with XWayland apps. For now, the only trick is to specify `GDK_BACKEND=x11` before launching Geany. Needless to say it is quite unpractical.