In the handful of 300TD's I've owned or ridden in the rear ceiling panel has always drooped in the middle right in front of the dome light, i.e. the panel that spans the ceiling and covers the speakers. See the pic below for what I'm talking about.

Is this by design (i.e. is something supposed to be there filling the gap in the middle), or is it just something that happens to these panels with age? If the latter, has anyone ever found a way to fix it?