Thanks for these informations I did not have because I did not succeed yet to make the new ADE version work with Wine. I nevertheless suspect there could be something else. The best is to provide an example.
Take this high-quality French ebook (a pity the author is no more active on this forum): Le roi au masque d'or.
If you download the full EPUB version (the one including I think Caslon fonts) and if you look at it with ADE 1.7 ("old" version) you clearly see that it make uses of thin no-break space
(for example before ; : ! ?) and ADE does not flag them with a question mark.
contrary to what it does to &# 8239;
But, open now this EPUB with Sigil 0.53. It will flag these same thin no-break spaces in Book view with a crossed rectangle (see Attachment) and ignore them in Code view.
It seems the book uses a different entity than &# 8239: to represent thin no-break space. If this is so, I would prefer to use it instead of &# 8239; because I would have ADE compatibility.