Fills of zones on all copper layers do not persist across reload

Description

The fill of zones that are on all copper layers does not persist through the file being reloaded.

At the time of saving a file:

After closing kicad and opening it again:

Steps to reproduce

1. Draw a filled zone -- set it to be on all copper layers (F.Cu and B.Cu in the case of a 2 layer board)
2. Fill the zones with the "b" keyboard shortcut
3. Save the file
4. Quit kicad
5. Open the file again in kicad
6. The zone fill has gone

KiCad Version

Application: KiCad x86_64 on x86_64

Version: 9.0.0-rc2-1.20250127git32673a0.fc41, release build

Libraries:
	wxWidgets 3.2.6
	FreeType 2.13.3
	HarfBuzz 9.0.0
	FontConfig 2.15.0

Platform: Fedora Linux 41 (Workstation Edition), 64 bit, Little endian, wxGTK, X11, xfce, x11

	wxWidgets: 3.2.6 (wchar_t,wx containers) GTK+ 3.24
	Boost: 1.83.0
	OCC: 7.8.0
	Curl: 8.9.1
	ngspice: 44.2
	Compiler: GCC 14.2.1 with C++ ABI 1019
	KICAD_IPC_API=ON

Locale: 
	Lang: en_GB
	Enc: UTF-8
	Num: 1,234.5
	Encoded кΩ丈: D0BACEA9E4B888 (sys), D0BACEA9E4B888 (utf8)