Serious alignment issues, twist in Y ?

[Qijian Neo]

Similar problem here, having 1390 machine. Always back right(BR) corner misalign vertically mostly. And once aligned BR, the front right(FR) misalign, the spot was low on FR
Still figuring out the solution.

One theory of shimming left Y rail not working is, the 2nd mirror is fixed to the Y gantry too.
If the beam still hit on 2nd mirror on spot along range of Y, no matter right Y rail is parallel to the left rail or not, the 2nd-3rd beam should also parallel to gantry.

Shimmed Y Axis.png

Here's the problem, I just make sure the 1st-2nd beam path was spot on 2nd mirror, in case of frame twisted, the beam actually not parallel to Y axis.
Refer picture below, the blue line means "calibrated" beam so that it hit 2nd mirror spot on in near and far end of the Y.
But since FR is high, the beam actually aimed lower in order to hit spot on. This causing the blue beam not parallel to the actual Y (left Y rail)

Twisted frame with misalign beam.png

So far that's what I am thinking of, if I figured out any further info, will shared here.

Any feedback on this is welcome