SUMMARY After editing a screenshot with the annotator and closing the editor window, the image gets automatically cropped (to approximately half its width and heigth). Using 2x screen scaling, in case it's relevant.
Seems to be an upstream issue, works fine with kimageannotator git master. Hopefully they'll release a fixed version before 20.12 release.
*** This bug has been marked as a duplicate of bug 426104 ***