eeschema: Links to hierarchical subsheets are broken after Altium import

Description

I noticed that in some designs, the links between the subsheets and the root sheet are broken.

The generated sheet file is called, for example, 6_power.kicad_sch. However, the root sheet refers to the file 6_power (without extension).

Steps to reproduce

1. Clone this Design https://github.com/dpmj/masters-thesis-lora-gateway-cubesat-payload-pcb
2. Navigate to the sub directory `carrier`
3. Import this design via Kicad -> File -> Import Non-KiCad project -> Altium design
4. Open the root sheet `carrier`
5. Navigate to the sheet `POWER MANAGMENT` (middle left site)
6. LMB click on this
7. An empty file opens instead of the current sub-sheet.

KiCad Version

Application: KiCad x86_64 on x86_64

Version: 9.99.0-unknown-1.20260131git2f0f5da.fc42, release build

Libraries:
	wxWidgets 3.2.8
	FreeType 2.13.3
	HarfBuzz 10.4.0
	FontConfig 2.16.0
	libcurl/8.11.1 OpenSSL/3.2.6 zlib/1.3.1.zlib-ng libidn2/2.3.8 nghttp2/1.64.0

Platform: Fedora Linux 42 (Workstation Edition), 64 bit, Little endian, wxGTK, X11, gnome-xorg, x11
OpenGL: NVIDIA Corporation, NVIDIA GeForce RTX 3070/PCIe/SSE2, 4.6.0 NVIDIA 580.95.05

Build Info:
	Date: Jan 31 2026 19:03:25
	wxWidgets: 3.2.8 (wchar_t,wx containers) GTK+ 3.24
	Boost: 1.83.0
	OCC: 7.8.1
	Curl: 8.11.1
	ngspice: 45.2
	Compiler: GCC 15.2.1 with C++ ABI 1020
	KICAD_IPC_API=ON
	KICAD_USE_PCH=OFF

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