Odd things are happening. Mine worked like yours. When I started Numbers it would bring up a Finder-like window that also had a "new document" button. Then I messed with the preference settings and now it does not, it starts with the template chooser or a blank document, just like the preferences say it should. File/New used to bring up the finder-like window, now it is the chooser or blank, depending on the preference setting. File/Open brings a finder window that does not have a button for "new document". So I checked my laptop to see what it does. That one brings up the finder-like window when Numbers starts and was bringing up the same thing with File/New. But then I messed with the template vs blank setting on it and now File/New brings up template chooser or blank document. Starting up Numbers still brings up the finder-like window. File/Open still brings up the finder-like window, including the "new document" button. Both are running OS 10.13.6 and Numbers 5.2. So I have two computers on the same OS with the same version of Numbers and the same preference settings and they are acting very differently.