I'm not sure there's a case to be had that if a person walked into a pre-cast and sustained AOE Illusion like Chaotic World or Mass Agony that they'd even be subjected to the spell. Ergo if someone inside the spell when it was cast fully resistedit, they needn't fear going outside then back inside the AOE either...
Up it a notch and use the example in the core book. Mana Barrier on the moving van. If the opposing mage had pressed thru the barrier once, then left the for some reason and then came back... Hopefully see where I am getting at with this. Like I said, seems like cheese, but then again, whats the point of being able to move the spell, since the taget leaves the area on their pass and the mage can't move it until their turn...
Mana Barriers are not Illusions. Apples and Oranges... and in this case it appears relevant to point out the difference. There's perhaps a surprising amount of significance to the rules given at the start of each spell type. The rules governing Illusions are on pgs 289-290.
Whereas something like an Invisibility spell absolutely "saves" data relating to the # of hits for the purposes of seeing if people who later encounter the subject resist the spell.. that's a case where those people aren't the subjects
of the spell. Like I said, it seems to be hazy and up to GM interpretation... but going by what I'm seeing I wouldn't have a spell like Chaotic World or Mass Agony keep afflicting new targets.. sustaining it only keeps it going on those who were initially hit and failed to resist. YMMV. OTOH something like Ice Sheet, an AoE manipulation spell, would absolutely affect new people as they walk into (well, onto) the spell's aoe.
To resynthesize: there are no blanket rules governing "aoe sustained" spells. It matters by spell type, and isn't going to always be the same answer across spell types.
OMG, do I have to prove I am human every time I post?
I think for like 10 posts, yes. But it'll stop relatively soon.