The AShooterGame_FreeForAll::DetermineMatchWinner function is set up to compare each player's PlayerScore with the BestScore variable to see which players are considered to be the "BestPlayer" which seems to be intended to be a high score system, as there can be multiple BestPlayers.
The problem is that BestScore is being set to MAX_FLT which is way too large for anyone to ever max or exceed with their score. The function ends up doing nothing as no ones score will ever qualify.
This code was the same in 4.19
Code issue, no steps provided - See description
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-64514 in the post.