MailDeveloper does a lot to mitigate any rendering issues across email clients. Sometimes it seems like MailDeveloper causes a rendering issue, but it is actually an issue which can't be prevented without restricting common use cases.
The hash sign "#" is a useful placeholder for links where an URL isn't yet defined. However, in some versions of Outlook any links with this href-value will be rendered incorrectly.
See this example of an incorrectly rendered button (Office 365):
The same button renders fine when it has a proper href-value set (still Office 365):