He struggles at the first sign of resistance.
He's mentally weak, has a limited skillset and is reliant upon the officiating dictating how successful he is because he resorts to brute force rather than any notable skill.
That's why his flaws haven't been exposed because he's played against two teams that can't provide much resistance to big men. Any starting level PF would be eating in his place too. And it's really less about the actual frontcourts, themselves, and more about how imbalance in a squad creates fractures elsewhere, because it forces other players to overcompensate in areas that they otherwise wouldn't have to and that affects the rest of their respective games.
Because Minny have an infinitely better balance throughout their team, in comparison to the Lakers and Steph-less Warriors, it's made it easier for him to perform. He has a greater platform to play off because he's already playing with a headstart. He's barely had to defend in this series because the sheer size and deterrence that Gobert, Naz and McDaniels are imposing on GS is already enough to take them out of the contest.
Then he can come down the other end of the floor against a mentally-defeated, smaller team, get a switch on a guard/wing player (or fatigued one), or someone like Looney or TJD, and half the job is already done.