Other people have kind of touched on this already, but clients prioritize connections based on stability and speed so things like seedboxes often get utilized more than your average home client to saturate a downloading connection.
I also noticed that my max upload speed is much less (e.g. 2MB/s) than what I get with speedtest (e.g. 7MB/s)
Again, other people have touched on some bottlenecks that can cause this (y'all are good lol), but clients also have a bit of connection overhead that will ultimately affect your maximum upload speed to peers at a varying degree. Here's some details, if it helps:
Overhead- Additional data used and required for communication and coordination between sender and receiver that is not part of the payload data actually being transferred.
You’re not doing anything wrong, the playlist function is just half-baked. My guess is they put just enough work into it to intend for it to be used in small chunks, such as a single evening.
Edit: I’ve just seen your clarifying statement, and although Plex playlist is relatively non functional I’ve never seen that specific behavior. Try hitting play on an item in the playlist instead, that works for me.