Hi,
When I load an image as a new reference layer in a document which has a double-wide pixels (2:1) aspect ratio, that ratio is also applied to the reference image. I guess it’s a bug, because the reference image should not be affected by the properties of the image you paint on.
The workaround is to shrink the reference image (width/2 in this case) and, since that reference has a high definition, it still looks fine when it’s stretched in the reference layer.
-lonestarr