I really don't think it has anything to do with playoff intensity. He hasn't looked right for most of the regular season as well (people who don't watch just didn't care because he was still randomly piggybacking numbers). Plop last season's version of Pettersson into the playoffs and I'm pretty sure he does well.
Definitely feels like there's some sports psychology issue with him in general (we saw some of that two years ago as well). It doesn't look like he outright doesn't care because defensively, he's still working and getting in the right spots.
This next bit is mostly baseless speculation, but he almost seems to exhibit post-concussion-like symptoms at times-- makes me wonder.