I think the problem is that clone should copy the area, but it does not do that exactly.
If you sample the top of the source area:
If you sample the top of the target:
Whereas if I take the demo image from the documentation, the colour is copied exactly:
https://docs.darktable.org/usermanual/4.2/en/module-reference/processing-modules/retouch/#clone-and-heal
