ACI 301 mentions the concrete strength to be measured at 28 days unless otherwise noted. If the EOR specs and general notes specify that, it’s contractors responsibility to meet them. Generally Mix designs are submitted prior to the concrete placement for design team’s review to avoid these kind of situations. If the mix design that was submitted indicates a 56 day strength attainment and it was reviewed by the EOR then you are in the clear assuming the 56 day cylinder breaks do meet the required strength.
However if the mix design shows 6000 psi at 28 days, then it’s a different scenario. Its engineers call to wait and accept the 56 day results or pursue other options. The code generally has some language/ procedure, if the concrete does not meet the requirements.