Trident: Y axis is uneven

[H4kunaMatata]

Hi all,

I have being dealing with this problem for a month now without success in solving it.

This is a 200x200x0.2 @ 0.2 layer height cube I print to check my first layer. 
Under normal circumstances, it looks normal, well, no anymore. See those gaps only at the very end??

 

I have disassembled my hotend in case something was loose, I cannot find anything out of place.
My z-tilt looks correct

The mesh for that sheet looks "alright"

 

I know this printer can print a perfect first layer whole bed, I only use that cube to check it.

It was suggested for me to loose y linear rails but how would this start out of the blue??
Would a damaged POM nuts cause that?? I can see the Z-axis turning during the print to counter-adjust ( z-tilt) so it is working but I wonder if the POM for some reason went bad.

I will be disassembling the chains to install umbilical mod so I am trying to gather as much information as possible to try/check at once.
I have spent weeks and lots of filament wasted printing this calibration test with zero success.

I mainly print at the center of the sheet so Idk when this problem started.

Thanks a lot

Edited November 11, 2023 by H4kunaMatata