Appending single copper layer board generated from gerbview wiped custom net classes and multi-layer zone settings

Description

While appending a board that was generated from Gerbview (only F.Cu layer used) erased my custom net classes settings and the multi-layer settings for all zones.

For net classes: only Default stayed unchanged (net class assignment patterns also stayed), but settings for each net class were totally wiped.

For zones: if the layer was set for L1, L2 and L4 then the inner layer (L2) was checked off; for zones that only existed on inner layers (L3, for example) nothing changed.

This wiping of settings happened without any user warning or confirmation.

Steps to reproduce

  1. Open Gerber with gerbview for front copper layer (*.gtl) - don't open any other layers on the same project
  2. Export to PCB Editor with the default layer settings
  3. Open a 4 layer project
  4. On pcbnew do File > Append Board and select the file generated from gerbview
  5. (pcbnew hanged here for a while and it looked like the file wasn't imported but was inserted way off to the left of the drawing sheet - something to be fixed as well, probably)
  6. At this point I saved the project and exited KiCad; when I reopened the project I noticed the net classes and zone settings were missing

Attached is the change log on *.kicad_pro

Screenshot 2025-10-23 at 12.26.38.png

KiCad Version

Application: KiCad PCB Editor arm64 on arm64

Version: 9.0.4, release build

Libraries:
	wxWidgets 3.2.8
	FreeType 2.13.3
	HarfBuzz 10.1.0
	FontConfig 2.15.0

Platform: macOS Version 26.0.1 (Build 25A362), 64 bit, Little endian, wxMac
OpenGL: Apple, Apple M4, 2.1 Metal - 90.5

	wxWidgets: 3.2.8 (wchar_t,wx containers)
	Boost: 1.87.0
	OCC: 7.8.1
	Curl: 8.7.1
	ngspice: 44.2
	Compiler: Clang 16.0.0 with C++ ABI 1002
	KICAD_IPC_API=ON