Digging in a bit. It looks a bit different than I thought it did on my end.

I periodically use Kodi to backup my metadata to an NFO file. But I guess I forgot to script that and haven't actually done that in a while. When Kodi scrapes the metadata it preprends the mpaa value with the string MPAA Rated . I did this to make it more obvious if the rating was scraped correctly since I've seen kodi attempt to scrape before but then just record the value as null.

1. Inside Out didn't have an NFO that Jellyfin was reading from. But Jellyfin is set to scrape its own metadata from all avaliable sources. I just didn't enable it to write its own NFO file since I want the files that Kodi makes to take precidence and not be overwritten be Jellyfin. So for the case of insde out, it's pulling the correct rating without any changes. As seen in this desktop screenshot.

2. A Boy and His Dog, on the other hand does have a nfo from Kodi and it does have that MPAA rating on jellyfin with the prepended string as seen in this screenshot and in this bit from the nfo file.

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<movie>
    <title>A Boy and His Dog</title>
    <originaltitle>A Boy and His Dog</originaltitle>
    <ratings>
        <rating name="themoviedb" max="10" default="true">
            <value>6.300000</value>
            <votes>309</votes>
        </rating>
    </ratings>
    <userrating>0</userrating>
    <top250>0</top250>
    <outline>A young man and his telepathic dog wander a post-apocalyptic wasteland.</outline>
    <plot>Set in the year 2024 in post-apocalyptic America, 18-year old Vic and his telepathic dog, Blood, are scavengers in the desolate wilderness ravaged by World War IV, where survivors must battle for food and shelter in the desert-like wasteland. Vic and Blood eke out a meager existence, foraging for food and fighting gangs of cutthroats.</plot>
    <tagline>A rather kinky tale of survival.</tagline>
    <runtime>90</runtime>
    <thumb aspect="poster" preview="https://assets.fanart.tv/preview/movies/4917/movieposter/a-boy-and-his-dog-5e3fe50ad3787.jpg">https://assets.fanart.tv/fanart/movies/4917/movieposter/a-boy-and-his-dog-5e3fe50ad3787.jpg</thumb>
    <mpaa>MPAA Rated R</mpaa>

3. Burning has a nfo file from Kodi. But the MPAA key is null in the NFO file. So there is no prepended string. However jellyfin still scraped the MPAA rating for it's self and correctly labeled it as seen in this screenshot and nfo.

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>
<movie>
    <title>Burning</title>
    <originaltitle>버닝</originaltitle>
    <ratings>
        <rating name="themoviedb" max="10" default="true">
            <value>7.400000</value>
            <votes>1440</votes>
        </rating>
    </ratings>
    <userrating>0</userrating>
    <top250>0</top250>
    <outline>Jong-su bumps into a girl who used to live in the same neighborhood, who asks him to look after her cat while she&apos;s on a trip to Africa. When back, she introduces Ben, a mysterious guy she met there, who confesses his secret hobby.</outline>
    <plot>An aspiring writer goes to the airport to pick up a high school friend returning from a trip to Africa but is disheartened to see her with another man.</plot>
    <tagline>The truth is all in your head.</tagline>
    <runtime>148</runtime>
    <thumb aspect="poster" preview="https://image.tmdb.org/t/p/w500/kXiF80o74fE9gf3Utf9moAI7ar0.jpg">https://image.tmdb.org/t/p/original/kXiF80o74fE9gf3Utf9moAI7ar0.jpg</thumb>
    <mpaa></mpaa>

So I guess I have 2 questions.

1. Does the string I put in the rating distort the recommendation system?
2. Does the recommendation system prioritize a local nfo file even if the value for the MPAA key is null when jellyfin actually has the correct rating in its cache?