Further reflection:
Why does this work? I mistakenly thought my mobile screen in portrait mode was 1024px wide, hence my choice of min-width as 1025px.
But it isn't: it's 1080 x 1920 px. And anyway, I had almost the same problem in landscape orientation, on both mobile phone and iPad mini, which has a 768 x 1024px screen.
So why does this min-width work at all on the mobile phone?
How does the server know it's a mobile if not from the screen width? I know the 'browser-agent' may tell it, but many phones now pretend to be 'desktops' don't they?
Not critical, now I have an apparent fix, but I'd really like to understand why it works at all.