If you have a gold badge in a tag then you can immediately close as a duplicate any question that has that tag. So for example I have a gold badge in [general-relativity] and [newtonian-mechanics] and should be able to close as duplicates any questions tagged with these.
However I just VTC'd as duplicate the question What would be gravity in one spatial dimension? and it didn't close. The question has the [general-relativity] tag, but it was added (before my VTC) by Qmechanic, so the question wasn't originally tagged with [general-relativity].
I also recently VTC'd as duplicate the question What makes neutron heavier than a proton? and it did immediately close even though it has neither the general-relativity nor newtonian-mechanics tags. However I note that it did originally have the newtonian-mechanics tag but this was removed (before my VTC) by Qmechanic.
So it looks to me as if the immediate close as duplicate code looks at the tags the question originally had, and not any changes made after the question was posted.
Perhaps this is just to stop me randomly closing any question by editing the tags before voting to close :-)