[AndreaSpooky]

I'm using a quite recent 8.2 build on x86
...
Have someone else this kind of problem?

Not been reported so far with these LE9.0 test builds so you may want to try it.

Testing with the latest LE 8.2.3 might also be worth a try as your "quite recent 8.2 build" covers both Kodi 17.5 and Kodi 17.6.

I was indeed on a December milhouse, and now I passed on the latest 9.0 build: issue still remains. Weird thing is UI recognise I have the subtitles file in the path, and even when I disable/re-enable/load it still doesn't render. The only way stays download it again o_O

OK, I kept trying, and this issue seems to be related to the way Kodi recognizes/catalogs the episodes or movies, and then opens the subs file in accordance to the filename (or DB name).
Example: I open an episode from the TV Shows listing, no subtitles shown, then I try opening the very same episode from the files section, the subs appears.