A while ago I complained about fish briefly swimming out of view in MA 2.6. I know they are supposed to, and some prefer that effect to make the tank seem larger, but if I had my way, the fish would never completely go out view. Maybe in that case the lionfish problem would never have developed.
I haven't noticed it with my lionfish, but I'll keep a closer watch on them now and report anything unusual. Meanwhile, owners should take Jim's advice and see how long it takes for the lionfish to reappear - if it does.