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 a bit longer on the Event Complete screen pictured above before clicking the "Start a new game" button. (2 minutes should be more than enough.) 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.