What causes the bug?
The bug occurs if you get to the "Select a map" screen before the prize from the Live Event is ready.
After finishing a Live Event, wait longer on the Event Complete screen before clicking the "Start a new game" button. This will give the game some time to communicate with the Fuel server and prepare your prize. If the "Select a map" screen doesn't say "Prize pending..." when you get to it then you should avoid the bug.