Which device are you on?My Music app never remembers which song I was listening to. When I go back in the app after having the song on Pause for a while, it just shows Not Playing. This is a little annoying.
I have seen this from the very first beta's of iOS 11: I think it's very annoying. I have a certain playlist I play in the car. Everytime I had to search again in that list where I was before, because at the end of the day it does what you describe.
This is one of the reasons I have stayed on iOS 10: it forgets where exactly in the song I was, but at least remembers where I was in the playlist. This way I can get in my car and get along with music. Otherwise:
1. no song plays
2. random song on phone starts playing.
I have an iPhone 6 btw, I blamed it on the 1 GB memory. Curious what device you have?
Edit: I see also iPhone X owners seem to have this. I presume this is absent then in 11.2.5