This playlist name thing I've also noticed.

More widely, many, if not all, of any recommendation algorithm in any field (music and beyond) are not 'intelligent' (meaning smarter than the right human) in any way whatsoever; as noted, usually some tag sorting going on - and where items have been auto-tagged vs manual makes no difference to the quality; you're still dealing with a programmed personality when auto-tagged.

However, they do (appear to) 'work' (exhibit intelligence) when it's performed on a topic of which you know little; there they offer a helping hand.

Interesting! Like what.. a genre name?

I named a playlist "metal", it started recommending metal songs without me having added anything yet.

That would be just one signal for the recommendation engine, given that you only named the playlist without any songs that probably weighed in quite heavily on the recommendations. Did it change after you added songs or kept suggesting metal songs?

Just tested it and yes, it keeps recommending metal even after adding a handful of obvious non-metal songs.