Project Manager: Sub schematics files still shown on the file list

Description

Sub schematic sheets are not hidden in my project and double clicking it starts editing a different sheet than the one under the main schematics. Also opening that sheet creates another project file (kicad_pro).

Is schematics files included in the parent schematics file with the new format, just like the symbols? Or is it still relying on external schematics file like the stable branch? If so why "enter sheet" and "double clicking on the sub sheet file in project manager view" shows different files? (Changed one's contents to see what will happen).

I'm not sure how reproducible this problem is. Because couldn't create it again even with a converted hierarchical project (complex_hierarchy).

KiCad Version

Application: KiCad

Version: 5.99.0-unknown-308ab8caf~106~ubuntu20.04.1, release build

Libraries:
	wxWidgets 3.0.4

Platform: Linux 5.4.0-56-generic x86_64, 64 bit, Little endian, wxGTK, ubuntu, x11

	wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
	Boost: 1.71.0
	OCC: 7.3.0
	Curl: 7.68.0
	ngspice: 31
	Compiler: GCC 9.3.0 with C++ ABI 1013

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=ON
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON