I've noticed that immediately after the match "Victory"/"Defeat" notification, there is a brief period before the winner screen shows up where you can still get kills on some players. You can confirm the kills with the sound that you hear when you kill players.
Do these kills count towards your match total?
For example, if I get 30 kills as Reaper and the "Victory" notification pops up, and before the winner screen appears I complete my 31st kill against a Mercy, does the game record me as having made 30 or 31 kills during the match?