I've had an occasional problem like it. More like oddities in preview, not a serious problem the seem to go away.
Brings up a question which occurred to me, may be irrelevant. If I use the Image Optimizer app separately, (I do) should the "automatically optimize while exporting" be unchecked, or does it matter?
And: do I need to be using the Image Optimizer app at all if it IS checked?
My theory: code that doesn't need to be there could be a problem waiting to happen.