Solved:
go here for the solution: medium.com/macoclock/how-to-add-device-support-files-to-xcode-ce4ca5965092
You have to download the ios 14 DeviceSupport files, unzip them, then move them to the appropriate directory. It is super annoying that XCode doesn't install all new versions when updating, but hopefully this saves some others an hour or two.
trancend
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Abnex's answer could have saved me over 24 hours of uninstalling/reinstalling/testing. After I built my archive in Xcode 11.5, I uploaded with Xcode beta 12 and it was accepted.
Super annoying glitch, but I guess that's on me for using beta.