Oddbean new post about | logout
 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