My desktop PC has a MSI MAG B650 TOMAHAWK WIFI (MS-7D75) mainboard, and every now and then it takes ~30 seconds until the BIOS POST screen shows up when turning it on. To solve this problem I decided to update the mainboard BIOS to the latest version V1.P3.
The update went fine, but when booting my Debian 12 installation I noticed that the Linux boot messages had the wrong resolution, and then the whole desktop environment was locked to a resolution of 1024x768 (4:3) - there was no option to change it via the Gnome Shell settings. The second monitor was also not detected.
I found the solution in a Linux Mint Forums thread: The BIOS update had re-enabled secure boot, and this prevented the graphics driver from loading. Disabling secure boot set everything back to normal.