Just am update. This was confirmed as a tech glitch and steps have been taken to re accommodate people in their preferred seat or preferred seating 'class'. If you have a premium seat and all premium seats are sold out before you/they realise there's a problem then you will receive a goodwill gesture. It's not ideal but only affect a very small percentage of passengers and as speculated earlier it was due to the default aircraft being a 319 on all routes.