The transform tool currently(this is changing…) paints the preview of the layer you’re moving on top of everything, not as part of the image stack, so that shows the contents outside the image boundaries.
The move tool doesn’t have a preview, so it only shows the outline of the layer you’re moving outside the image bounds.
It’s always been this way, except for a very brief moment in 2006 when I made it possible to paint layer contents out of the image bounds, but that was quickly killed by, of all people, the KWord maintainer (Krita was part of KOffice back then).