The first thing that comes to my mind is to boot from a (recent) live iso, reinstall the grub, and try again.
You can use the tools in the live US Garuda Welcome or:
Check also that Secure Boot on BIOS is still disabled. You never know with windows…
Edit:
Careful here 
(disable bitlocker from windows first, if renabled after your tests)
With a quick search on the Arch Wiki, I found this possible warning (I don’t know if this applies only to Surface laptops):
https://wiki.archlinux.org/title/Microsoft_Surface_Pro_3#Disable_Secure_Boot
And maybe also consider:
https://wiki.archlinux.org/title/Dual_boot_with_Windows#UEFI_Secure_Boot