Hi Damien,
I thought something similar - but it's Quality 10 in PS and also tried 87% in Lightroom which is roughly equivalent.
Since I posted this - I emailed the files to someone else to check on their PC - and it also looks OK to them.
So, I think it has to be something to do with the viewer/screen on the other devices.
Thanks, Frank
OK - just had an idea. I was looking at the files via the Dropbox viewer. I loaded them into snapseed - and they look just fine. So, it's the viewing software in Dropbox.
Never had problems with it before - but it must be the background.