M420 Z Option Not Working Correctly

Firmware
1

I have submitted an issue request to GitLab, I am posting this here in case anyone has seen this before.

The M420 Znn option for fade height does not work as expected. Any value other than 0 will result in the z axis of the object being squished. I am using Lulzbot Marlin 2.1.3.0.21 on a Taz Pro using the Universal BL Touch firmware. Problem exists on at least the previous version as well, 2.1.3.0.20.
Test object was a 28mm cube. Setting M420 to Z10 or Z20 resulted in object being squished by 2mm in Z (resulting in a 28x28x26 object). Z10 caused the first 8mm or so be squished, with the remainder of the cube being normal. Z20 caused the first 18mm or so to be squished with again the rest being normal.
Thinking this was related to my Z axis stepper driver, I completely replaced the Archim2 with a brand new one. Problem still persisted, so my next set of diagnosis was mechanical. Nothing could be found there. So obviously something software was causing this.
See attached photo. I have marked in black where the squish stopped. Left cube used M420 S1 Z10, middle cube M420 S1 Z20, and right cube M420 S1 Z0

image_50373377
image_503733771920×1440 90.1 KB

2

If your bed and gantry are not coplanar, this would be expected behavior, as it’s fading the adjustment over those layers.

Post the output from the M420 V1 command so we can get an idea of what corrections are being made by bed leveling.

3

Taz Pro homes the Z at the top, so it should be level or pretty close to it by the time it starts printing.

Here is the output:

Send: M420 V1
Recv: Bilinear Leveling Grid:
Recv: 0 1 2 3
Recv: 0 +1.582 +1.717 +1.609 +1.520
Recv: 1 +1.597 +1.695 +1.584 +1.535
Recv: 2 +1.685 +1.759 +1.590 +1.432
Recv: 3 +1.691 +1.699 +1.451 +1.283
Recv:
Recv: echo:Bed Leveling ON
Recv: echo:Fade Height OFF