Desktop lock after using Spectacle

After using Spetacle a couple of times the desktop locks (no response) and a hard stop (shut machine off) has to be done to clear problem. Occasional it will work for several saves off of the screen, but most of the time only one or two.

Running 19.08

Very weird indeed. I fail to reproduce this for now.
Is the systems kernel frozen? You can check if it reboots by pressing
Ctrl+Alt+Print and typing reisub.
If it reboots then it is not a kernel freeze but seems to be an Xorg freeze which might be caused by the graphical driver for your graphics card.