I had a long and friendly back and forth with support, was asked to do a firmware update, to perform manual tweaking with the nuts under the bed (Manual Bed Leveling / Manual Bed Tramming | Bambu Lab Wiki), send numerous log files but ultimately they only ever saw a variation of 0,2mm and I kind of gave up explaining the problem.
Here is a video (I also sent to support) showing the movement of the lead screws.
Bambu Lab bed bow - YouTube
To my understanding the screws have a pitch of 8mm and I see close to 45 degree movement (so that would be more like 1mm instead of 0.2mm.
I cannot really explain why I see so much movement on the screws, see a big gap between the bed and a staightedge but only 0.2mm show up on their logfiles. …other then the carbon rods are bent the same way - which they are not as far as I can tell.
anyhow: as I said, I kind of gave up and figured that the X1 is still a excellent printer for the money and I have other means of producing parts if I really need the precision. …and maybe it even is preferable to have a bow in that direction to counteract warping 
edit: wrote 4mm pitch leadscrews before - I have to check again when I´m home maybe they in fact don´t have 8mm - so that would cut the error in half. but still…