If htey are assmebled and interfaced together properly...
Assembly is made by the subsystem supplier.
If there is an interface problem I suppose there will be a system malfunction on the ground.....and the hull/system has passed the test flight and many more hours.
After all Sukhoi consults with Boeing on design, manufacturing, certification and quality system, supplier management and in addition Boeing is a representative to the technical board.
Me thinks it's human related unfortunately.