@Charlie_Barham, thanks for the details. I believe this is the same bug raised by @Mike_O that the Sonos queue is not cleared when another playlist is selected, and this new playlist will just be added to the end of of the existing queue.
@Mike_O has reported this behavior to SmartThing support and SmartThings support team did confirm it as a integration bug. See below for Mike’s post in the SmartThings community, and I’d suggest you to post your comment in the thread, and open a SmartThings support ticket as well to help SmartThings prioritize the tasks.
I’d guess the shuffle is enabled so it kind of rotates between two playlists every time you hit next.