How to keep buried deck posts from rotting
I'm answering from Australia so building practices may be different.

I'd call your builder and insist that he come back and install the footings properly. Post brackets are expressly to keep the posts above ground so the bottoms do not rot.

Installing them below ground is not a very good idea.

Edit

This need not be a difficult fix but it is my strong view it is the builder's responsibility. I'd wait for a time when the ground is reasonably dry. It will be a pain in the butt.

Instead of replacing the footings:


Support the deck on jacks / trusses.
Dig out around each post footing until the top of the existing footing and the bracket is exposed.
Cut off the bottom of the post at the desired height, 4" - 6" above grade, or in line with local best practice .
Unbolt the cut end of the post from the existing bracket.
Obtain and, to the top of the existing bracket - bottom of the post, bolt an extension bracket. If some diligent fossicking at the local hardware store does not turn something suitable up you may need to get them fabricated. They should be externally and internally galvanised(a).
Into the existing footing drill 4 x 5/8" holes at a 6" pitch ~12" deep.
Smack a 2'(b) length of 5/8" rebar into each hole.
Box the top of the existing footing to the appropriate height.
Apply concrete.
When cured release the support jacks(c).


a.  This is a must. Copper Arsenate is a typical pressure treatment. In direct contact with steel (iron), depending on the pH of your soil, weird electrolytic things can happen.

b. The top of the rebar should be ~2" below the new top of the footing. 2' is a guestimate. Adjust appropriately.

c. Take some care with this. The post bracket extension must be installed firm so that the deck does not sag when the support is released. You may need to shim. A small piece of fibre cement sheet works a treat.

Good luck.