Uneven Z motors speed - lead to X carriage not straight (no replies)

Hi all,

I have a problem with my printer.
It's a custom P3Steel 30x30x20, with a SKR 1.3 / TMC 2208 UART / Marlin 2.0.2 (updated few days ago), with 1 stepper motor for Dual Z axies motors, in stealchop mode.

My problem is that the Z axies motors doesn't seams to have the same speed. It's very hardly noticeable, doesn't impact on small height print /move, but as soon as i make long prints, with some heigt, i can clearly see a the end of the print the left side of X cariage (the one with the X motor) lower than the right side.
So i have to manually change the height of one side (left for example) so the X axies come back straight.

you can see in attached picture, the motor1 is slower than motor2 (Z axies), which lead to X cariage not straight after 1 or 2hours of printing

I have try to change to voltage in the configuration_adv, but couldn't notice a difference

Any idea from where it could come? bad motor? bad stepper? marlin bug?
I'm really out of ideas...

thanks for your help