Uneven Z-axis tramming along X-axis, help needed


I read through tramming posting but couldn’t find my case, so posting for help.
My router is not perpendicular with respect to Y-axis which seems require shim according to what I read here and there.

Problem is, if I move router to all the way right (X max), router is perpendicular but if I move router to all the way left (X zero), there is big gap on square on the bottom, i.e. it’s tilted towards to Y zero. Of course, this gap narrows as router moves X+ and disappear at X max.
And this is kept along Y movement, i.e., router is perpendicular at both Y0 and Ymax when X max but has gross the same gap at both Y0 and Ymax with X 0.

The only explanation I can imagine is there is twist in X bar but I can’t imagine how and if so, how to fix.
The machine is almost disassembled and reassembled on flat (cast iron table saw top with <0.005" flat) to make sure frame is not twisted. I matched diagonal length to make sure frame is square.
Would anyone can point me where to look to fix this issue?
Thank you very much in advance for all advice!

Are you sure there is not a twist in your base/waste board?
If you have a digital level use that to check both at each side.

Thanks for the reply.
That was the first thing I suspected when I found this issue.
So, I moved machine to my table saw’s cast iron top which I manage it flat to 0.005" and found one side was rocking if I push. So I removed waste board, loosened bots, clamped down base to table saw top, and tighten bolts. Now all rockings are gone and stay flat but the issue is still there.

Ok what about a digital level?
I would really be surprised if the square extrusion had a twist in it!

For the record, this issue is resolved.
Giovanna suggested to disassemble maker slide and put on flat surface to see whether there is twist.
It is INDEED twisted. I can’t imagine how that thick extrusion is twisted but it is. I got this machine as second hand and there was a lot of abuse sign on it. So there is good possibility.
Bought new maker slide with chrome marking and replaced old one. And issue is all gone.
There is some Z-tilt left but this is addressed with shim behind of router attachment as well known fix.
Now it trams X-Y axis with very light ridge only can tell with finger touch. I may be able to make this gone with more shim but it’s already more than enough.
Finally working.