Why do some admin users get the old interface but some get the new? And this is inconsistent. Some admin users get old interface in "Add group" and some get the new. However, everyone appears to get the new staged interface for "Add user". Everyone has been using the new admin centre for months, if not years
What is the correct name for this new "staged" interface within the new admin centre?
As with many new features, they gradually rollout the bits and it can take weeks or even months for the release to complete, so it's kind of expected. And sadly, so are bugs, as QA hasn't been a priority for Microsoft for years now...
You should just use the Feedback button in the AC to put all this info in, nobody from Microsoft will read it here.