I've had this issue before but never really investigated it further until today.
I use a lot of polygonal viewports to show twisting routes of water mains along a section of an OS map and add connection details in the remaining paper space areas.
What I've noticed when using this method is that OSNAPS still snap to the map features in the empty space, however, only within a rectangular area that would fit the extents of the polygonal viewport.
I'm not looking for a fix as it's not a major issue, but just thought I'd report it and see if anyone else has noticed it and knows why it happens?