New Login UI session cookie doesn't survive browser close

Today I figured out that the sessions cookie set by the New Login UI doesn't have an expiry date or max age and thus is created as a session cookie. For Firefox (with restore session on) this means an endless living cookie, but other browsers (at least Chrome and Safari without restore tabs) are clearing the session cookies at different times. This means that I have to login after closing my browser even if the settings are to stay signed in for 10 days. Is this deliberate or a bug?