OK, I have a much more refined diagnosis.
It’s only Safari on the iPone 5S that experiences these issues. I run other browsers (Chrome, Firefox, etc) onmy 5S and it’s not a problem, it displays as expected. On my daughter’s iPhone 6S, no issues. Old iPad no issues. A couple of android devices no issues.
iPhone 5S running the latest IOS (10.3.1) model MLMA2LL/A – this is the only device I have that experiences all these weird and bad behaviors.
And I must say, the Clean Education theme’s responsive display is much nicer than Jetpack!
So, for now I’ll just ensure I test on a differnt browser!
Thanks, Mahesh for your quick response – sorry I should have checked these other things before – live and learn!