skipped first layer of different color on multimaterial, then prints in mid air, then below the previous layer ('no sparse layers' enabled)

[Ro3Deee]

Description of the bug

I make a simple STL file to test MMU filament change:

On PS 2.6.0 alpha3 (0.4mm layer height with 0.6 nozzle), first layer of different color is skipped, but Z is raised:

The layer is also skiped for suports:

And the second layer that is printed (the third if you count the skipped one) is printed below the previous layer (also for supports):

On PS 2.5.0, first layer of different color is skipped, but Z is not raised:

Expected result (sliced without painting):

Workaround

Disable no sparse layers

Project file & How to reproduce

Here are the files:
skipedLayer.zip, includes:
MMuTip.FCStd , MMuTip.stl , MMU-Tip.3mf , expected_slice.png , painted_object.png , sliced_after_painting.png

project with supports (painted) :
MMU-Tip-with-supports.zip

Checklist of files included above

• Project file
• Screenshot

Version of PrusaSlicer

PrusaSlicer-2.6.0-alpha3
PrusaSlicer-2.5.0

Operating system

Ubuntu 22.10

Printer model

Original Prusa i3 MK3S+ MMU2S

[halofx]

I am seeing a similar issue when using No Sparse Layers, where the first layer is shifted up, and overlaps the second layer.
Print preview does show the problem. This happening with 2.6 alpha 2, and 2.6 alpha 3. I am on Win10.

0.20 QUALITY setting shown. 5mm boxes, right box raised .5mm.

0.2 layer is empty.

First layer is printed at 0.45mm height.

Second layer is printed at 0.4mm layer height, and subsequent layers print as expected.

[Ro3Deee]

I have an example where the first layer after the last toolchange is made, is skipped (it jumps form Z22 to Z23).

The following 3 layers are printed below . Here is the picture (fan speed view):

Project:
Prusa MMU2 idler body - MOD Ver AA.zip

[kubispe1]

Thanks for your issue, the fix will be part of the next release. Closing issue
Commit: 026ca7b
SPE-1458