My first thought was substandard Chinese steel. I once heard an interview with someone at a large construction firm who talked about how the Chinese manipulated steel testing results. But it sounds like there are a lot of other potential causes.
My first thought was Chinese H1B PhD structural engineer, but this looks like a handoff FU between the delivery crew and the building crew. It is like the delivery crew said “we are leaving and taking our jacks/supports with us”.
174ft long 32 ft wide 800 ton slabs of concrete don’t do well only supported on the ends. Steel wouldn’t either and steel is stronger than concrete in bending, torsion and tension... all things that happen on a partially build bridge.
You leave the supports in until the cables are attached or the arches are complete. Bridge building 101.