In that case, there are only a few things I can think of that may be at play here:
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Again, that randomness bug
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This (resolved) bug related to similar artists: Mood and Style tags suddenly disappeared from music library
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The size of your library: are there enough tracks to build a sufficient radio?
Out of interest though, where did you read the recommendation to store your files under genre folders? Plex’s recommendation is usually to use Album Artist > Album > track
