26.4 beta and RC versions are unable to be created on anything but 26.4 beta host OS

We're trying to create 26.4 beta and RC VMs on 15.x and 26.3 host OS' without success. We see

Tue Mar 17 17:27:36 40 anka.log (install) 45803: failed to install macOS: Error Domain=VZErrorDomain Code=10006 "Installation requires a software update." UserInfo={NSLocalizedFailure=A software update is required to complete the installation., NSLocalizedFailureReason=Installation requires a software update.}

Yet, if we create it the same way on 26.4 beta host OS, it works.

We've tried the usual tricks of installing latest Xcode and preparing it (accepting license, etc). But, they don't work on 26.3 and 15.x.

What's the trick to get the creation of 26.4 to work on <= 26.3 host OS?

26.4 beta and RC versions are unable to be created on anything but 26.4 beta host OS
 
 
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