afraid to mess up their engagement statistics

I think it's this, plus more general algorithm nonsense. Except since it's YouTube, the algorithm's constantly changing and no one even knows how it truly works, so some of the logic behind these decisions might not even be valid anymore.

Yes, I have heard a couple of youtubers saying that each channel must have a singular theme otherwise the algorithm won't know what's the target of each other video.

For example there is a yt channel called sabine hossenfelder in which she makes videos about Science, but from time to time she posted some videos of her singing, but when she posted it, youtube wold recommend it to her regular viewers and they would watch less then the usual video so the algorithm was recomending her channel less because of that, so in the end she created a new channels only for the those music oriented content