I have a 12 year old house, with the original roof. On one side of my
roof, there is a sunken rafter which is causing a dip in the roof.
I was told that it would not move or sink anymore, but I would love to
get your opinion on this matter.
1. What caused this to happen?

My guess is that the framing member wasn't nailed down properly if at
all. Has any other part of your house shown the problem? Any problem
with the ceiling inside? If not then it has to be the framing.

2. How severe is it?

I think it's pretty bad. Residential roof are always wildly
overdesigned and failures are rare even in old houses. That house is
pretty new and there's no way that is right.

3. Is it likely to get worse?

I don't think so. Since the problem is at the top of the house there
is no weight bearing on it save the deck and the shingles. Even if the
rafters are on 24" centers that leaves adequate support for now.

4. How common is this problem in other houses?

I have been up on few roofs as a roofer and haven't ever seen one like
that.

5. Any other comments or advice?t

Just document the damage and wait until the roof need to be replaced.
Then you will be able to do something about it.

Claims on the builder can be difficult to pursue but something worth
considering. If you can bring a claim against the builder right away
they might agree to address the sitiuation when the roof is replaced.