In fact the "panning while zooming problem" makes it impossible to use Reaper for serious video editing anymore, as I need this very often. That's a bummer because I like the concept of editing video like audio.
But what I didn't know is that it behaves differently in other color spaces. How could that even be?
I will test that in the next days. But it still needs to get fixed somehow, because its very obvious especially for people who come from other video editors. I was pretty surprised when I realized the first time how bad the interpolation works. There must be something overlapping between pan and zoom. I don't know.