Spotify 'closing' when left running on locked iphone

[senzavolto]

Hi, I usually always have Spotify open and running in the background on my iPhone 6 and my Bluetooth on as I get in and out of my car for example. So if I get in the car and press play on the stereo it should control Spotify immediately. However very regularly the app has 'refreshed' (or closed or crashed or something) when the phone has been locked (or I've been doing something else with it over a period of time) so the controls on my car (or any other Bluetooth for that matter) don't work - I have to take my phone out of my pocket and go into the app and select something to play again.
This is very annoying! Every so often it actually begins to control the music app instead and plays the 1 song I have stored in that 😉
I'm sure that it's only been this last 6 -9 months that it has done this and I wondered if anyone could help - am I missing some setting somewhere?