Big Sur & Bad Magic!
I experienced the Bad Magic iboot flag error when trying to update my 2018 mac mini (that i just bough in July from apple). to big sur.
I have reset nvram, erased a boot camp partition, erased the drive and attempted internet recovery, attempted to install via flash drive, in the end, I had to install Catalina, which I actually got installed. Still wont let me upgrade to Big Sur though.