Ubuntu 20.04 server under Hyper V

[Charles M]

We have some Ubuntu 16.04 - 18.04 server installations running in
Hyper V on Windows Server 2019. None of the installations seem to have
any problems displaying in Hyper V.

Something appears to have changed in 20.04 because although I can
install it, when it boots I only see a black screen - it appears to be
some graphical text mode. I've tried options like VGA=TEXT but this
hasn't worked.

I know this isn't strictly a *buntu issue, but since previous versions
of Ubuntu didn't seem to have this issue I thought someone might have
some suggestions.

Our installer is PXE-based and we've used almost the same options we
did for previous versions.

Thanks for any suggestions,

Charles