This issue started happening to me a few days ago.
I am using iPhone SE 11.4, the latest available update I believe.
But to be clear, the issue was occurring both before and after I updated to the latest software.
It was in the native music player I first noticed. I thought maybe some of the music I transferred from iTunes had a bug or something! But then, I tried Spotify and noticed it was happening while playing music on this app too.
What is so infuriating is that it seems like music playback is working totally fine - then, I lock the iPhone and put it in my pocket, and begin walking/going about my business, and then the start/stopping begins happening. As well as this, Siri is appearing on and off though I haven't requested her!
Currently disappointed and disgusted with Apple. (Have been an iPod user for years - but this is my first iPhone)
Have an upcoming trip this weekend and not looking forward to being unable to enjoy music while travelling. 😟