I have recently switched to macOS Sierra and discovered that using C-x 5 2 to create a new frame works just fine, i.e. no different than OS X, if you do not enter full screen mode (via the green window decoration button). If you are in full screen mode C-x 5 2 is ignored, but if you then exit full screen mode it is clear that the second frame is now a tab (macOS Sierra tab feature).

I have not seen this behavior mentioned but then I haven't been paying that close attention.

Prior to macOS Sierra, in full screen mode, C-x 5 2 would create the new Emacs frame in a new OS X window.

Seems to me that this behavior in macOS Sierra is an Emacs bug to the extent that it will take Emacs code changes to achieve OS X behavior. Is anyone aware of a fix for this issue that has not been released yet? Or is it work not yet taken on? Or am I completely wrong?