Here it is from MotM:
View attachment 251306
and the "older version" from SCAG:
View attachment 251307
I would agree in general, but in these cases he is not "approaching" after being hidden, he is attacking at range. Crawford even said that a hidden creature can move 5 feet "into the open" to make the attack and still have advantage.
Given the feature's Malek has (racial camouflage, expertise in stealth, reliable talent) and cunning action to Hide, and that he has enough movement each time to move out of LoS, and then pop back out... I think Hiding is okay--after all, this is what he is
good at.
Now, yes, Vecna
might have seen him move behind a pillar, but he doesn't know which side he'll pop back out from? So at best 50/50, you know? With 3 other PCs, I would say Vecna is "distracted". YMMV, of course.