I had not noticed this behavior prior to the newest release. Drop shadows on layers elsewhere in the documents are causing boxes in the top left corner, size relative to the size of the drop shadow. There is nothing in the vicinity of the box.
In the sample below, the drop shadow on the text center right is causing the box. Turn off the shadow and the box goes away. There are no hidden elements on the pasteboard to the top left—the drop shadow is on that text group only.
Checking Preferences>Legacy Compositing makes the problem go away, but I'd consider that a bug more than a feature.
Photoshop 21.0.1 Mac OS 10.15.1 AMD FirePro D300 2GB