I dont know why you are having this issue as I dont with my FZ10. I have a sunpak 383 and use it extensively for macro work, using aperture priority the camera still gains up. I agree that when using the complete manual mode that gain up doesn't happen but with A mode it does. I am confused.
Odd. Mine darkens in aperture priority compared to P mode. Maybe yours is a later model. I ordered mine as soon as it was released. It is probably a little late, but maybe I should look into it.
Edit: I just ran some test shots and the shutter is extremely low in low room lighting with AP. I think I would get a lot of ghosts trying to handhold at 1/3 second.
Manual is as bright as aperture priority if I run the shutter speeds down to the 1 to 1/3 second speeds that AP uses. Still not as bright as P mode though.
If I just set 1/60 in shutter priority it is as bright as aperture priority and would automatically go to F2.8. Unfortunately there are some ranges that f2.8 doesn't work too well with my flash. And I still can't see what I'm shooting very well. It is brighter than the f4 or f5.6 and 1/60 in manual that I usually use though.