

I can boot into Windows through the BIOS using only the MS-signed bootloader instead of GRUB or any chain loader, and Windows itself considers Secure Boot to be enabled successfully.
I assume that’s because your motherboard still has the microsoft keys installed besides the MOK keys, and it verified the bootloader with that. thats why it accepts the ms signed bootloader. as I know not all motherboards allow removing it, and there are a few buggy ones that get hard bricked if you do that.

I think there’s some linux command to query the installed keys, but here I have only found the command for listing all the installed mok keys: https://wiki.archlinux.org/title/Unified_Extensible_Firmware_Interface/Secure_Boot