(R)-2-Methylcyclohexan-1-one is oxidised with mCPBA. Take the migrating carbon to be the secondary carbon (C-2) and note that CIP priorities at that stereocentre keep the same order before and after reaction. The product is

In the Baeyer-Villiger migration the migrating carbon moves with retention of configuration. At the stereocentre the substituents in priority order were C(=O), ring CH2, CH3, H, and after the reaction they are O, ring CH2, CH3, H, occupying the same spatial positions. The descriptor therefore remains R in (R)-7-methyloxepan-2-one.