That definitely looks like a Cinnamon bug, specifically its screensaver component. Worst case they’ll direct you to where to report the bug, or they’ll move the ticket themselves.
Also, obligatory this is one of the many ways X11 is insecure and unsuited to proper screen locking. Lock screens on X11 are just fullscreen windows you pray the window manager won’t ever allow to unfocus, close, resize or move or not cover the rest of the desktop.