I've configured the dns records correctly to use sub domains for both 365 public sites. Most of the time it works fine but occasionally, 365 redirects to the default domain (ie domain-web.sharepoint.com) instead of the sub domain. It then keeps returning the default domain for no apparent reason.
So there's something wrong in the way the servers interpret the http request or some kind of hidden redirection. Neither can be fixed by users.Since there's no rhyme or reason as to when it happens it's very hard to pin down what the bug is.
These are the sub domains:
Both are registered in the account and set up with A records pointing at the 365 ip address. So there's absolutely no reason for the default domain to be returned EVER.