The TMDB scraper in Kodi retrieves the very first line (earliest release in the user-specified country) for a movie, and grabs the certification from that line. The "Premiere" line seems to often have no certification entry, thus leading to incorrect information. Perhaps the scraper should grab the "Theatrical" or "Theatrical (limited)" release line, but that opens up another can of worms for the scraper programmers.
Regardless, "Premiere" showings almost always have ratings attached to them, since the content is usually identical to that in the "Theatrical" release. So, the "Premiere" releases should have certifications in the data here.