Hi Micha, what do you expect in those areas?
I did a quick test with two surfaces AND the split curve is shown in white:
(I guess this is what you see as well, that the surface edge and the curves aren't identically optimized)
And here the curve is hidden:
And here it is shown and hidden again (point is that either the green or the red edge is showing):
As you can see the surface edges are drawn in the same manner, but the curve isn't using the same optimization settings (if any).
Edit: This was odd, viewcapturetoclipboard seems to use a different draw order than the viewport it self... or something is messing up the paste here as I can't capture the reversed draw order with viewcapturetoclipboards, so I used another tool: