Sleep timer while listening to music

I enjoy listening to music while I fall asleep. Under the clock app, you can set timer to 'Stop Playing' when timer ends. Brilliant. I've set it to shut down after 30 mins.

Except it's inconsistent. Sometimes there's no issue, other times I'm woken 2 or 3 hours later by Music still playing, or EarPods alerting me to imminent powering down. So I again set timer and fall asleep. And repeat.

I use either EarPods or Haylou-GT1, depending on which have power. The Haylous have better sound quality and are more comfortable, but shorter battery life.

I cannot figure what's causing the inconsistency. Setting the timer, before I run the Music App or after? does skipping tracks affect the timer, or incoming texts, emails etc? Changing headphones and re-pairing issues?