Here's a potential work-around for those of you getting disconnected, like me, at the end of the last race.
once to Horizon Life
On the bottom center select Horizon Solo (if it's not there you're already in Solo)
over to Online Adventure
Select Quickplay Adventure (big box in the middle)
Select Free-For-All Racing
Complete the fifth race
The game is having problems switching players from the Quickplay Adventure to Online Horizon Life. This bypasses that and will hopefully give you a better shot at not disconnecting after the last race.