Circles not round (flat on 2 sides) CoreXY custrom printer

Hello,

I have a custom made printer with CoreXY mechanical and build volume 50x30x25 cm.
It is built on Hiwin and rock-solid construction but still, there is a problem with not round circles.

A was thinking that it is backlash effect byt tightening/loosing the belts done non results.

Attachment 12658 Attachment 12659

But the circles are flat on 2 sides (kinda straight lines in X-axis direction). Is it something that I can correct by firmware?

One more photo with dimension and drawing :
Attachment 12660 Attachment 12661

Thanks in advance.