This is an old trope, but about 70% of the time, when a team beats another team twice in the regular season and then faces them in the playoffs, they beat them the third time too.
It's not that I think the Panthers aren't any good; clearly they have a pretty good team, but they don't match up with the Saints very well at all. Their offensive line is weak whereas the Saints defensive line is strong; their receivers are weak whereas the Saints secondary is strong; their front 7 is big/physical but the Saints offense gets the ball out quick and to the edges to negate that.
For some reason the Falcons, even though they are not nearly as good or talented a team overall, present a much tougher matchup for the Saints than the Panthers do. They just match up a little better. It's weird.