It depends on the circumstances. For instance, in borderline cases, where the design validation outcomes are not so conclusive, I would have recommended a separate design validation review, to allow a thorough consideration of whether to move forward to design transfer, to repeat the validation with required adjustments, to redo the design, to update the requirements specification or otherwise. A separate design transfer review could later follow, for ensuring that the design was appropriately transferred and regular production is ready to commence.
Combining reviews along the design control flow is like stating that all but the last phase combined do not have critical exit gates. Otherwise the risk of redoing a phase is too big IMO.
Cheers,
Ronen.