Never had a phone mute the music in headphones when I put it on vibrate or silent. This is with Spotify at least. Any way to change this? I only can listen to music in normal mode.
Does it happen with any other app besides Spotify? You could try booting into Safe Mode and see if the problem still happens when using the stock Music app. On most Android devices, while powered on, press and hold Power until the Power Off menu appears. Press and hold the Power Off selection until the Safe Mode prompt appears. Tap OK.