It is common. Unfortunately fixing it while not expensive, involves quite a bit of work. You'll need to cut and shape some new foam. I had a comprehensive thread about taking the seat apart and I did a partial replacement.
The downside is, you'll not get the original "feel" of the seat. It's constructed of latex foam with a series of holes moulded into it. The thinking behind this is to keep the foam "firm" enough while allowing a controlled amount of "give" to accommodate the shape of the passenger. It also means the original shape is retained and the foam doesn't compress so much over time (clever thinking in the P6 extends even further than is obvious).
Maybe this can be done with different densities. The chances of finding a good, original replacement are also fairly slim but might be worth a shot.
But yes it's not so difficult to do.