Because 4K is very new and not mainstream yet real-world hardware has not converged on standard resolutions like FHD yet.

I have a QHD+ 3 200 x 1 800 laptop

It's customarily plugged into a UHD 3840 x 2160 screen (via HDMI)

In that configuration, the kernel does not choose the correct mirroring setup on boot. It will choose a mode, where the external screen, only mirrors the top of the internal screen (so the bottom of the internal screen is not displayed).

This is very annoying because I basically only need the console to debug boot problems, in rescue mode the text size on the internal screen is too small to be comfortable (without manual adjustment) and the external screen only works for debugging as long as the console does not enter the hidden non mirrored screen section.

00:02.0 VGA compatible controller: Intel Corporation HD Graphics 620 (rev 02) (prog-if 00 [VGA controller])
	Subsystem: Hewlett-Packard Company Device 82b9
	Flags: bus master, fast devsel, latency 0, IRQ 130
	Memory at a0000000 (64-bit, non-prefetchable) [size=16M]
	Memory at 90000000 (64-bit, prefetchable) [size=256M]
	I/O ports at 3000 [size=64]
	[virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
	Capabilities: [40] Vendor Specific Information: Len=0c <?>
	Capabilities: [70] Express Root Complex Integrated Endpoint, MSI 00
	Capabilities: [ac] MSI: Enable+ Count=1/1 Maskable- 64bit-
	Capabilities: [d0] Power Management version 2
	Capabilities: [100] Process Address Space ID (PASID)
	Capabilities: [200] Address Translation Service (ATS)
	Capabilities: [300] Page Request Interface (PRI)
	Kernel driver in use: i915
	Kernel modules: i915