I can get this to happen if I brush with the centre of the brush just off the canvas. It looks like it then treats the brush centre as a new "tile" of the image so if I brush along the top I am using the bottom of the new tile.
See below which is taken from an image with a dark top edge and light bottom edge
Brush centre just inside the image - no impact
Moving the brush centre just off the canvas and I pick up the image from the bottom
So the answer appears to be keep the brush centre just inside the image.
Edit to add : It only happens with the Impressionist setting checked
Dave - much obliged. Thanks very much. If I select the layer I am painting into would that stop the "overbrush"?
Surprisingly, to me anyway, I tried "select all" on the layer to be painted earlier - fully expecting that to work. It didn't
Yes, I just tried it too and the brushed pattern still spills. Hmm..