Just doublechecked: Both my J1 and the unused replacement PCB i have would fail miserably in that case - the centre is 1mm higher than the sides, both in X and in Y direction there. And the five metal clamps then even it out against the glass plate 
Maybe folks having trouble have badly bent clamps? I’m struggling to help on this one. I’ve had zero problems with leveling my bed, and rarely even need to do it, it’s so stable. I feel for people if it doesn’t work since it’s one of the things I’ve enjoyed about my J1. And for sure, leveling was not a strength on my first printer, even when it had ABL that always screwed up my offset.
Same here. Recalibrating is something I almost only do after removing/inserting the hotends, and I need the 0.2 nozzles maybe once a year or even less.
Bent clamps are at least something that could explain the topic - with the bed PCB shaped as it is, you will have the effect that a bent-open clamp will rise the edge where it is and subsequently lower the diagonally opposite edge. But I would think the user should be able to notice that…? On the other hand I have long learned I tend to overestimate the level of comprehension of basic mechanics the average person has… probably an “occupational disease” 