Not that it means too much that PSG messed up this game after having that 2-0 lead, but it does show how carelessly PSG can play and give up a comfortable lead in a game in their own stadium because they don't care. If this was a game that would have decided about who becomes the Ligue 1 title winner, would they have played the same way?
This is what I am criticizing about that league, PSG knows that no matter how this game ends against Stade Brest, they will win the trophy anyway. But in the Champions League they want to be different and then they realize that if they don't have to care at all for 38 games in Ligue 1, out of a sudden they have problems to do it better in the Champions League. It will forever be one of their deficits to be playing in a weak league.
PSG entered the dressing room by scoring two goals before the end of the first half, they almost won the match, they had this thought while going to the dressing room, this thought made them lose in the second half. What happened in the second half was when Brest started to play more willingly, PSG's entire game plan became dysfunctional and they couldn't even maintain the lead.
It was a pretty bad forty-five minutes for PSG in the second half. Mbappe played well but it wasn't his day. He tried hard but it didn't work. If he had been on that day, PSG could have saved the day.