I've had this happen on multiple Ubuntu installations on different hardware now. When I take a screenshot of an area of the screen with Ctrl + PrnScr, about 50% of the time after drawing a rectangle on the screen, the screenshot never goes off. You can tell it never triggers because the rectangle doesn't flash. It may happen for fullscreen screenshots too, but I use the rectangle one 95% of the time, so I can only confirm it happens with that.
I've noticed this seems to happen when I do it quickly, but if I create the rectangle, pause for a second, then let go of the mouse button, it seems to work more often. Any resolution for this issue?
This AskUbuntu question from 2 years ago sounds like the same issue, but the asker is trying to resolve a slightly different problem. A relevant piece of info to this issue is that Ubuntu at some point switched from using the Gnome Screenshot app to using a daemon to take the screenshot when you hit PrnScr. Among other things, the daemon doesn't support a default save location, so this just might be another bug hanging around.