Did you asses the floor’s ability to support heavy new cabinets and stone counters before you did this remodel? Did you reinforce your joists? Old houses should be done settling by now. That’s sagging is a red flag that you have a structural subsidence that needs investigating. Call in a structural engineer since your contractor obviously didn’t investigate anything when he had the chance. .

The remodel was done by the seller (they flipped the house) and because the city was suppose to inspect the structure of the house I had hope it was fine

This is also a three family ( up and down) and all three kitchen were done with granite countertops at the same spot (the three floors have the same layout)

This is no more indication of structural failure than a hangnail is of a broken finger. Splashes, cabinets, and tops moving independently of each other are common.

Remove and reinstall the splash please.