It doesn't make any sense that the pool sunk after the water was removed or that adding water back into it would make it magically rise again!

It sounds like your PB made a critical error and did not dowel the decking into the pool walls. Typically when a pool is poured some of the vertical wall bars are bent down so that when the decking is poured it is tied into the pool structure forcing the two to move together structurally. Depending on the age of the pool it may not have had those bars in the existing decking, or they may have been sawcut when the old decking was removed. If that was the case, they should have drilled holes into the pool walls and epoxied dowels into them so that when the new decking was poured it would be tied to the pool. Also, do you know if they put a sand cushion down under the decking? If they didn't and you have soils with a moderate to high clay content and that clay gets wet it can expand and lift the paving with it.

Thank you so much for your time and consideration -
There is no way I would have known about that tie in.

1) They did put down sand under the rebar - I'm doubting the rebar was tied in.
2) We do have a high clay content (we call it Gumbo - it acts like a sponge when wet and dry)
3) I don't know about the previous (if any) vertical wall bars.

Can the deck be ripped out and done correctly now? Can they, drill holes into the pool walls and epoxy dowels into them so that when new decking is poured it is tied to the pool? Or is it too late since the new plaster has been poured?

We have a lot of clay here too, it's really a pain when you go through a drought followed by heavy rains because it can move a lot under those conditions. If the decking is demo'd they can indeed drill dowels into the pool walls, it won't affect the plaster because they're drilling into the back side of the walls. Unless they use a bit that is too long and it goes all the way through!

Here are some pics so you can see what I'm describing, in this first picture they've set the steel and about every 36" they left a rod extending vertically. They will later be bent down and will be cast into the decking.

In this picture you can see that they've bent the bars down and then placed additional reinforcing for the deck over them. This was after it had rained and the bars were partially into the sand, but before the decking was poured the bars were placed on chairs to raise them above the sand. Ideally the reinforcing should be centered in the decking slab. You can also see in this picture where they installed dowels into the existing sidewalk in the bottom of the picture. They drilled into the sidewalk and installed dowels about every 36". This is similar to what they should have done on your flatwork.

This is a finished pic, mainly to show the mastic joints. There should be a mastic joint between the flatwork and pool coping, and also between the new decking and existing decking or sidewalks (if there are any). In our case you can see (barely) a mastic joint on the backside of the coping and also where the decking meets the existing sidewalk (existing sidewalk is to the right, it's a slightly darker color).

Make sure you don't drill into the pipes in the pool either. Most of the the pipes should be below the Bond Beam...but just an FYI to think about when you start drilling.

Not sure what happened but I would not think the pool would lower without water...which is a lot of "weight"...if anything it would rise. Same thing if the shell floated while it was empty.

My guess is that your pool "was" tied to the deck and when you left it empty it floated your deck high and then when refilled the pool, the shell went back to its normal place...leaving your deck lifted.

Did you get a lot of rain while the pool was empty?

I'm not sure how you can fix that without repouring the deck...maybe cutting the rebar away would be the answer. Does the deck look higher...get a level out there and check and see if it elevates more toward the pool than it use to.

Ok, maybe I misread...
This is a new deck that was just poured while the pool was empty?

tresw,
I beleive those lengths of rebar are more for bonding the patio to the pool as opposed to structurally tieing the pool shell to the decking. Actually, I was told that you want the pool and decking to be separate in case one shifts a bit, and the other doesn't. Then you will not get cracks, due to the differing movement rates.
Now, this theory may be totally different if you have a cantilevered deck. Maybe, in this case, you do want the pool and decking to be connected.
I am just a pool owner and am only passing along what I was told by a couple of pool builders. I am curious as to what the experts say.