Well that was 11 years ago, and the issue is in the OP "so I killed Geany" ... same problem, but none of us seemed to notice back then.
But losing scribble is the least of the potential issues you can have when killing Geany, no modified buffer will be saved, so any edits will be lost, recent changes mean that session and settings are reasonably safe unless you are really (un)lucky and the crash happens in the middle of saving them.
If you have autosave on you might again be (un)lucky that the crash happens during a save and you lose the original file too.
The moral is "just don't crash it, its not written to handle that".
Geany certainly could be improved to to save modified buffers intermittently separate to the original files like some other tools do (Vscode for eg), but thats a reasonable amount of work and "somebody"s got to do it. And since it involves startup and reloading it might not be possible in a plugin, it might need to be in core.