Unexpected overall page tint

Very occasional InDesign user here - and I'm scratching my head over this one...

InDesign 15.0.1 Windows.

I've built a 28 page A4 document. Colour used throughout. 3 master pages, 1 layer in each. Master pages just have a running header, master text frame, and footer. A handful of  PSD format illustrations in the document. In summary: nothing complex.

Everything looks fine on screen. However, when printed every page - whatever the master page - has a background tint - a significant tint around 30% of a warm yellow. EDIT: I should also say that I've printed this on two completely different printers - both with exactly the same result, so it's not a printer issue.

There are no visible frame / boxes on the page and the tint extends to the full page area excluding bleed.

I exported this for print - and the pdf is fine, both on screen and in print - no background tint!

Any clues on what I've done here?