this might be the exact same issue you are having?

https://www.reddit.com/r/arch/s/xkL9lMWy0q

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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