The dataset is not “jazz,” it’s Ultimate Guitar’s tablature of jazz songs, so the data is very low quality.

Yes, I think you're right, many Ultimate Guitar charts are generally low quality. I remember once checking out a song in F major and the IV chord was notated as A#, LOL.

To be fair there are also high quality charts on UG, but they generally require a paid subscription (I think they are called "pro" charts). Maybe the dataset should have been limited to those but I'm not sure about the legal implications given that they are a paid product.