this might be the exact same issue you are having? https://www.reddit.com/r/arch/s/xkL9lMWy0q
Looks similar, but no simple fix.
Are you able to boot into it and reinstall grub?
Yes, but the devil’s in the details. Doesn’t recognize /boot as vfat.
Post on the Arch subreddit and hopefully someone is able to guide you in the proper direction