Why are my bottom and top paddings behaving differently?

In our designs, we use zero-height (or width) containers quite a bit, because absolute positioning often results in unwanted positioning (rounding to next full px-value).

Until now, it was no problem to use bottom padding to “push” objects, that are nested within the zero-height container, “up” into position.

This does not seem to work anymore. Why? Did I miss something? Thanks for any advice!

I hope the screen-grab explains my issue. Top-padding (left) behaves as expected, but bottom-padding does not.