Unfortunately, my multi-monitor system is still running 10, so I can't do any experimenting to see if I can duplicate your issue.
What I did do is search the Feedback Hub, and...
Found that apparently, Microsoft has a lot of work to do with their new "Snip & Sketch" app.
Some of these seem to be similar to what you are describing and there seems to be some consistency in the problem
It's associated with a laptop using a larger second external monitor and even more so if the external monitor is being identified as the primary monitor. Here's one such example.
There are others that are similar. Unfortunately they don't seem to be attracting much attention from "on high"
The occasional Up Vote or two, but very few with even any comments.
This was the solution to the issue I was having. I switched my main display to my laptop, which is vastly smaller than my external monitor. I am fine with doing that now that I know this is the reason for the snipping tool problem. Thank you for this information. I am now happy!