During these last days, I have been working on some cases that our customers are able to see master database where before they didn't see it. This is due to a changes done in our platform and, for this reason, we have included a new functionally in the Portal to show or not these hidden types. This hidden types are some ancillary resources are created/managed by Azure infrastructure and the Portal does not display these resources by default.
If you don't want to see your master database in the database list, please, review the option that you have, for example, in the All Resources view, like this one below. Toggle this checkbox if you want to show/hide them.