Assuming the car data functionality that can be displayed on the infotainment screen in the Golf works the same way as it does in my Polo GTI, the long term data information resets itself back to zero after 100 hours of driving time.
Are you sure yours didn't reset itself 300 miles ago, which could account for the step-change in the long term mpg figure. Has your driving during the last 300 miles been mainly short journeys which could account for the lower long term figure? What is the 'number of hours driven' time shown as within the long term vehicle data? If it's low, then this might confirm the car has reset the data. Suggest you manually calculate your mpg for the next couple of tanks of petrol to see what the true mpg is.
As far as the tyre pressure warning is concerned, it may be a coincidence that has happened a day after your car was at the dealer. Was one or more of your tyre pressures low compared to the recommended pressures, hence the warning being triggered? Are your tyre pressures normally constant at the recommended pressure when you check them regularly?