Lol. Funny that it's not just me who has this problem. However, I have given up trying to solve this problem. But since the proportions in my views usually don't change dynamically, I went the easiest way. I made the radius of the rounded corners a little smaller and then centred the element above it. Within certain limits, this even works with the Designer (calculation of width and height of the underlying element and corresponding centric positioning of the element above). However, if the radii of the corners are too large, this no longer works.
As in your picture, it just looks ugly. It simply disturbs the visual aesthetics of the view.
It would be helpful if a 'childview' would automatically adapt to the rounded corners of the 'parentview' below.