How old is the bass now?
(Have a look at the label inside.)
Just curious: who built the bass?
Due to the photos the sunken part is on the E side, not the G side.
I have the same situation on mine. The amount very similar, maybe even more.
I went to one luthier for inspection and talked to one more who gave me an explanation.
First thing to check is the bass bar. The ends may have become loose. I used a small webcam on a stick and a led chain to make photos from all sides over the whole length of my bass bar, but in my case the bass bar was OK.
(Link to photos at the bottom.)
The first luthier showed me more drastically sunken tops that he needed to replace by a new one. They were more on the thin side.
(My bass is a heavy built 4/4, 110 cm string scale 5-string. The top is about 8 mm thick at the f-holes.)
But he said that this would not be needed on mine (yet). Just having a look that it doesn’t get worse.
BTW, when I visited him my bass was about 30 years old. The sinking process on mine was very slow.
You might want to ask the seller if it was slow (over several years) and maybe how long it stayed at the actual position.
Since I didn’t know the real reason, I installed an adjustable saddle (Moser HVU) myself to take some pressure off the top. It has three positions, the lowest a bit above the original one. I tried all three positions, the lowest very similar in sound and volume to the original one, the middle OK but a bit weaker and the top one dropped the volume too much. So I’m using the middle position but can change to the lowest if I think I need it.
It might not have been needed, but it was rather inexpensive and I could do the work myself and it would be no big task for a luthier either.
Then I talked to the second luthier (I didn’t had the bass with me as I wanted to show a product demonstration, mainly for someone else only.
After the demonstration I told him about the situation of my bass and he said that the top most probably shrinked because of the wood which might have been too fresh. Shrinkage is mostly perpendicular to the grain, so the smaller width of the top pulled the top down. This process happens mostly during the first (ten?) years when the wood changes chemically (which a third luthier told me), so if your bass is older it might stay as it is.
The third luthier that had my bass first (but I didn’t realize the sunken top as a potential problem then) told me he needed to shorten the soundpost a bit (and set it a bit further away from the bridge foot as it was too close). That might have been a result of the shrinked top as the soundpost didn’t allow sinkage on the G side and got more pressure on it.
The main reason to repositioning the soundpost was getting a nicer arco response and it also helped that single element pickups got all five strings equally well. Before that I needed two element pickups.
So it might depend on the thickness of the top (how thick is yours at the f-holes?) or if the sinking continues or not. Measure the distance of the top height at the f-holes once a month and write it down. You may have measuring errors, but over a longer time you we see if it goes further down or not.
Just a reminder: check the bass bar!
This is most important.
If you cannot do that yourself or are unsure about the diagnosis have a luthier check it.
Anything else written above is irrelevant if the bass bar is not fixed over the whole length.
Here is a link with the photos of my bass bar which is well attached and the top view at the f-holes:
www.dropbox.com
Good luck!
BTW, your bridge seems to be bent backwards (toward the tailpiece) which is rather unusual. Typically it be is the other way round. Steaming and clamping might bring it back to shape as it is not too much yet.
