That was IT, Sir Van Nuffel! Well SORT OF.
I don't know how it ever got set to 'Tile large pages' - and the page preview ~looks~ suitable for a single page. But when I set the Scaling to 'None' the Print Sheets changed to 1
- what I wanted
. Then, though, the Print button threw an alert to try checking Auto Rotate because 'some pages ... are outside of the available print area'. Pages, plural.
Anyway I cancelled, checked the box, and then Print got a second alert, same 'available' issue but this time suggesting setting the printer preferences to a different paper size. It was set to Letter, the only paper I have. So I decided to Print and IGNORE the alert, and it printed fine. With no extra sheets.
This reminds me, when it was set to Tile and was printing 4 sheets for one page, the extra 3 weren't totally blank. They each had a narrow portrait-oriented 'gutter' (or whatnot) containing a small edge portion of the doc page - even though the preview showed the whole page. If the preview panel had shown an edge being cut off I'd have had a better clue. Maybe.