bootable partition ang problema mo.. kung nka MBR ka at UEFI settings ng bios mo ganyan ang problema na lalabas. and vise versa.
one way to fix it is use dedicated GPU boot in windows and convert partition.
never touch the BIOS. unless the MOBO manufacturer itself is telling you to. if it works leave it be. because lately marami na akong nababasa dito using the world "Flash BIOS" like its the normal thing to do. its not.