I was hoping they'd fixed this! The "lifetime mileage" appears to increase linearly with distance after it reaches 1024 liters (270.5 US gallons) accumulated fuel use, but stops increasing when it reaches 32,768 km (20361 miles) traveled since reset. Or something like this. It looks like floating-point underflow (additional quantities are rounded down to zero before being added to the cumulative total).
It might be interesting to let it run for a while longer to see if it stops increasing (it did this at 76.2 MPG for me). You're almost there!
The reset instructions should be in your Multimedia manual. For MY 2015, it's on p.164, in the 'Information' section.