It's happened to me. Eventually, I found a box ticked telling the capture area to follow your mouse-pointer - how it got ticked, after I had used various versions of Debut for years and never even knew it was there, I do not know. But once it was unticked the problem disappeared.
Now for the bad news - despite trying to find the offending tickbox for the past hour... ...
Got it! I was looking at too old a version, this is a newer one - open Selection tool:
Hover your mouse over the highlighted box and a popup will say something like "Select area under mouse pointer"... something like that. Probably that's got activated somehow.
If that box is not in exactly the same place as in my snap, it's probably because it was tinkered with for a later version.
