(When it has started in UEFI-mode, the MBR drive can't be bootable, and BIOS mode can't boot to GPT.) Windows has never been designed for dual-boot really, even though the BCD makes it look like it ...

Many high-end games, professional software, and cloud security applications now require Secure Boot and TPM 2.0 features, which are only supported when the system disk uses the GPT partition format.