No. I'm actually looking into it now, but I don't have a whole lot of time so I suspect I'll get nowhere. Issue is likely in Scintilla IMO. Happens even if I use a clipboard manager and can see the current clipboard within it! Happens even if I close all Geany instances and open a new one and try and paste into that.
I wonder if it is related to https://bugs.gentoo.org/189230 (which is https://sourceforge.net/p/scintilla/bugs/626/).