1. Injuries forced a change of system and PSG's defense suffered from it
PSG announced hours before the game that the defensive trio of Marquinhos, Achraf Hakimi, and Nordi Mukiele wouldn't make the trip to Brest with the rest of the team. For a squad already missing center-back Presnel Kimpembe and a coach locked into using a 5-3-2 system... something had to give.
It was Galtier who conceded, using a flat-four on the defensive line featuring Nuno Mendes and youngster Timothee Pembele on the full-back positions along with veterans Sergio Ramos and Danilo in the middle of it.
El Chadaille Bitshiabu stayed on the bench providing some cover in the event of another injury, and that put Ramos a notch ahead instead of playing his best role: deep-libero in a three-CB line. That, along with the presence of the inexperienced and underdeveloped Pembele caused Brest to break for an offensive counter-attack that ended in their lone goal and was this close to prevent PSG from getting their 21st victory of the season.
The 4-4-2 have worked better if/when all pieces have been available (including Neymar, Marquinhos, and Hakimi/Mukiele), but it is just impossible to expect good results using it if Ramos/Pembele are together at the back. The former is overcooked and only works in a five-man defense and hiddend in the middle of the three center-backs these days. The latter is still clearly undercooked.