If I press the play/pause button on my Bluetooth headphones the music stops as expected but it does not resume when pressing the button again.
This also happens when using Siri: The music pauses but it does not resume.
To resume playing I need to use the Roon app.
I’ve tried this with 3 different headphones and the issue occurs with all of them.
I don’t have this issue with other music apps such as Tidal or Apple Music.
Any ideas?
Thanks.
iPhone 13, OS 15.2.1
I’m also experiencing this problem, and it’s really annoying. Did you find a solution?
Unfortunately not.
Roon’s iPhone implementation isn’t great.
I have other annoyances too that don’t occur on my Android device.
Buy even with wired headphones, Roon downsamples the audio on Android despite it being a 32/384 + DSD device. 
I gave up on Roon for mobile and switched to using Plex as a DLNA server.
I experience this too with my new Sennheiser Momentum 4 headphones. What’s weird with the Sennheisers is if I take off my headphones it auto-pauses, and I put them back on and it auto-resumes with Roon fine. But if I tap on the ear cup to pause - it pauses fine, but tapping again to resume does not. That’s why I thought it was a Sennheiser software issue, but maybe not. It only happens with the Roon app (all others behave as expected from my experience).