This is not about selecting the "all" option in the new navigation when filtering for multiple tags, it's about the old tag links for filtering questions by multiple tags.
You used to be able to filter questions by multiple tags by having a url like
/questions/tagged/first+second
Which would trigger a search for [first] [second], which meant that only questions with both first and second would be displayed.
In the new navigation though, this request will perform the same search with one small change: by default, the questions will be filtered down to questions containing first or second. This means that any existing links for filtering questions by multiple tags will no longer work when the new navigation is released to everyone.
I confirmed in chat that this behavior still happens for those without the new navigation. Those with the new navigation cannot test this as an anonymous user as only a single tag search is allowed there.
