From all the luthier threads I have read on here about putting a dome (radius) on tops and backs, I gather that some amount of deflection is normal on a uke with a flat top. Along with other desirable characteristics, the domes keep the top from getting those deflections, or at least more noticeable ones. The strings put a lot of tension on the bridge and want to pull the back up. If it is glued well, about the only thing that can give is the top, which eventually (sometimes years) take on a dip in front of the bridge.
I don't know anything about Kamakas, but I have heard Pete Howlett say that most inexpensive Asian ukes will take on this set because they have flat tops. I know my Ohana SK-35 has a slight dip in front of the bridge. As you say, its really only noticeable if you get the light just right, or are looking for it.