There is no obvious reason for this as I have set this projector up similarly and have maintained VERY solid focus even at the full extent of the zoom range.
What I would ask is whether or not you have used any keystone correction at all. You should not have used any keystone correction, but should have maintained a perfectly level projector which is square to the screen.
The follow up option/idea, even though it is a pain, is to pull the projector down and to set it on a table towards the center of the screen height and to see if you have the same issue.
Realistically, it sounds a bit like you may not have something square. The projector should be perfectly level - not tilted down at all.
The screen should be vertical.
If this is the case, I would expect the projector to perform very well and be extremely sharp with all test patterns.
If not, then I would be calling Panasonic for warranty issues.