I think in the playoffs teams have been playing him a lot closer/more aggressively and with a plan to have layered help as he tries to navigate in the offensive zone. Guys stand up to him rather than back off and give him space because they know if he beats them individually, they have backup from another teammate. Teams will just collapse down on him in the offensive zone in the playoffs.
His biggest flaw here IMO is not quite recognizing that quickly enough or trying to ram through the same kind of plays over and over...he and his linemates need some better plans or passing options for him to take when a team starts trying to collapse on him. There should be some players open, he just needs to move it to them more successfully. If his line starts burning other lines more often that way then they'll back off the focus on Panarin as much and he'll get a bit more room.
Since he doesn't exactly have a standout physical skill like amazing skating but also loves to always have the puck, it's a big harder for him to generate that space when a team tries to step up their aggressiveness on him and can gameplan around that
At least IMO. I don't think he was particularly doing anything "wrong" last year in a lot of ways, he was trying to shoot and not be a perimeter player but teams really focused on bottling him up. So, again, he and his line need to work on some ways to beat that regularly