SUMMARY
I (try to shift to digikam to) rate my images in the "preview mode" (since in slideshow mode, I can't zoom to 100% to evaluate the image).

Rating works as expected: hit 0-5 to set the rating.
Except, when an image was already rated to the same rating that I want to give:

Pressing a "3" on an image that already has three stars will store a 0-star rating.  (Same for other ratings, as well, i.e. pressing "2" on a 2-star rated image, ...)

This also happens in overview mode, where it is less of a problem, because there I have visual feedback about the rating.

STEPS TO REPRODUCE
0. Enter full-screen and preview mode
1. rate an image
2. rate it again with the same value
3. observe that the rating is "zero"

OBSERVED RESULT
Image is not rated anymore (0-star)

EXPECTED RESULT
Image is rated with the key that I pressed.


ADDITIONAL INFORMATION
If there was the same little info-box as in "slideshow" on the bottom left, I had the necessary visual feedback needed to avoid ending up with awesome images, erratically not rated at all.