Hmm, while I generally always choose a custom username, it also strikes me that I always manually add an email address too. So I’m not sure
I can offer that Virtualmin 3.62 is a bit older, that’s from September last year. If the problem you’re having a bug, that could certainly have been resolved by now.
You might consider upgrading Virtualmin, Webmin, and the Virtualmin Framed theme to the latest available, and then trying it again.
-Eric