Frankly saying it is a Hybrid Partition Table. While it is GPT, it has an MBR overlay. Windows likes to mix that stuff for legacy reasons. That way you can boot in BIOS mode while it is a GPT partition table on Windows.
Grub on the other hand needs just a seperated 10MB partition where it writes the MBR, no hybrid table needed.
This partition has the equal propose as a grub partition flagged with bios_grub, so that an UEFI can boot in BIOS mode on a GPT scheme.
You need to verify if efi files for Windows are written there plus an entry on efi boot loader. All that said, you would need to reinstall the Windows Bootloader in EFI mode, but no support here.