If works proceed there is a chance that the completed works may result in a non-conformance.
Not sure you are obliged to, but if you don’t proceed and you are late it may be your liability if you are then late.
There would probably need to be more information needed to answer this question specifically. Why would they need to give you an instruction at all, unless it is a change to the Scope, in which case I don’t know why that would be Contractor liability.
If the works may result in something that would not comply to the Scope, then it should be clear who’s liability that should be. The Contractor should then make a decision accordingly as to whether or not to proceed.
What I can say for sure is that the Contractor is obliged to follow an instruction given by the Project Manager (clause 27.3) unless it is illegal or impossible,