First, thanks for reply.
Second... I don't understand the relationship.
I presume this refers to PS' background save function that lets you continue working (whereas before you'd have to sit and twiddle your thumbs until the save process was complete)? Why would that have any impact on how Lightroom views the file? It's a save one way or another. And it's a fairly poor tradeoff to give up that background save functionality (if I'm understanding correctly) so Lightroom has a proper representation of the file. Also doesn't jive that this behaviour would impact a top level masked layer (my vignette) but not a merged up layer.
Need some more background here.
Please read the below link confirming this PS bug. I agree it makes no sense, but that's what Adobe says is the only solution until they issue a PS update with a fix.