Happening to me too - skipping through songs, playing when closed, opening voice control. I got my phone replaced today by apple and despite this it is STILL happening on a brand new device, can you please provide a solution?
I have an iPhone 6 with iOS 11.0.3 . I deleted Spotify two days ago and re-installed it.
I'm still having the same problem...the music skips when I'm listening to it via Bluetooth in a vehicle. I don't use Siri at all, and while it happens when I'm using a headset and walking, it also happens in the car when I'm listening to it over its audio system (2014 MB ML350; 2014 Nissan Altima).
When I use it in the Altima it won't play at all via Bluetooth, even though the screen shows it's playing, and indicates the correct artist and song. If I use a cable/plug it in to the USB connection, eventually I can get it to play. If I connect my wife's iPhone 7+ via Bluetooth hers plays perfectly.
I'm using Spotify Premium, which I love, but this is really annoying.
Again, I have an iPhone 6 with I have an iPhone 6 with iOS 11.0.3; I have 18 playlists, and am using the most current version of Spotify, 8.4.25 .