I have Geany 1.33 installed on Windows 10.
When I copy text in Geany via context menu and paste it in Geany, it works. When I copy text in Geany via ctrl-c and paste it in Geany, it works. When I copy text in Geany via context menu and paste it outside of Geany, it works. When I copy text in Geany via ctrl-c and paste it outside of Geany, it DOESN'T always work.
I didn't experience this behavior with any other application installed on my Windows 10 system. I also didn't experience this behavior with a Xubuntu system I'm using.