I have a document open in Photoshop on my secondary monitor (DELL UP2516D). As soon as I click cmd+ or cmd- to zoom in or out, the document jumps to the MacBook's monitor.
I'm using Photoshop 19.1.0 in Standard Screen Mode on a MacBook Pro (15-inch, 2017), 2.9 GHz Intel Core i7, High Sierra 10.13.2.