As far as I understood this was a regression from 5.12.8 and fixed with 5.12.9.

See commit https://github.com/Alinto/sogo/commit/be440baa

Yes, the false representation has been corrected with 5.12.9.
Now, nothing is highlighed/marked in the matching mails.

Hello, yes the subject has no more highlighting. But the body has.
It's no ideal but it was the fastest way to fix the problem with the time I was given. I assume, as a subject is only a few words, it will ok if it was not highlighted anymore as long the body still have highlighting.

Maybe I'll come back later with a fix for that (or if anyone else is interested to make a PR for it ^^)