Brendan:
Finally found the true fix.
Yes, it never had anything to do with AAM - barring its choice of fonts for the site, which are common enough. My intention is to provide a solution for people using Ubuntu on their home PC or indeed using any other op system which doesn't carry fonts like Segoe UI, Helvetica (Neue), etc. Perhaps even some who like me had a font in this list installed with faults in it.
The CSS file gives a group of fonts for rendering the text, i.e. 'Segoe UI', 'Helvetica Neue', Helvetica, Roboto, Oxygen, Ubuntu, Cantarell, 'Fira Sans', 'Droid Sans', sans-serif.
The browser will endeavour to find Segoe UI on the user's system. If it can't be found, it looks for the next one on the font list, then the next and so on till it finds one that the user has natively. In my particular case, the first one found was Helvetica. But for some reason this font was not working right, it was showing accented vowels as Cyrillic characters.
My solution was to download
Segoe-UI-Linux font from GitHub and install manually.
Please delete previous solutions which are limiting to user experience.