From the point of someone who has worked in a materials lab for the last 35 years, I’d be asking if there were any grounds for rejecting the specimens and results. It is amazing what you can find when you read the standard test method. Whilst those grounds for rejection are meant to be reported, they rarely are, in fact, and if they are, the import of them is lost on most people.
The above method also requires that if abnormal results are obtained, the specimens shall be broken open and inspected.
If the specimens are only (say) 7 day ones, a decision needs to be made as to whether the risk of carrying on is worth taking. This is something to be discussed with the structural engineer. Hopefully, they are more familiar with the concept of strength development and what affects it than what they may or may not have learnt in their concrete labs way back at uni.
As notes in their answer, there may be scope to justify the concrete, after a little “sharpening of the pencil” by the designer. This would not be the first time that one such would have said “oh, look at that, it does not have to be that strong after all!”