New web page opens in docked Web App instead of Safari proper, bug?

I make 'web apps' like described here

Turn a website into an app in Safari on Mac – Apple Support (UK)

So I have in the dock, a web app for Google Maps, a web app for Facebook and so on.

But when for example the web app for Google Maps is the front app, and I click on a link in an email I received, the page is opened in that Google Maps web app, instead of in Safari proper. This seems like a bug to me. The pages of a Safari web app should not be replaced by an 'outside' url.

A web app should be limited to open only pages of a certain domain. (like maps.google.com or facebook.com), agree?

Or is there a way to prevent this from happening?