FZ200 Viewfinder Quality Focus Issues?

Pretty happy with the FZ200 but the viewfinder seems questionable?

I AF focused on a distant object then used the diopter wheel to fine tune for my vision and it looked sharp, at least in the center.

But often I'm finding I often need to adjust the diopter settings again.
Also the edges viewfinder of the look distorted compared to the center area.

Think its a defective viewfinder or just it's limitations ?