First of all, show us the results. One cannot diagnose that which one has not carefully examined! You may well have an inferior lens copy on your hands, or it may be any number of other issues. Or maybe there's no issue at all! I see no point in throwing speculation at you. *Iff* you show us the negs then we may be able to say whether it is a DOF issue.
Second, to test a lens properly, you need to remove DOF and vibrations and such from the equation. You need to test a subject that has details that are within the DOF, and you need to use a tripod, and you need to use MLU etc. You need to be sure that the same type of film sees the same exposure and development too.
Third, compare apples to apples: enlarge the 35mm negative to the point that
the print is the same size you'd get from the medium format print. Indeed, if you loupe a 35mm neg and a medium format neg side by side, you may well see slightly less edge sharpness on the medium format neg, especially toward the corners. This is no surprise; there are few medium format lenses that can compete with good small format lenses right across the frame (if you don't believe me, then shoot 35mm through your medium format lens. In fact I have personally used only one medium format lens family that pulls off a 35mm crop well, namely the mamiya 6 and 7/7ii RF lens family). Overall, you need to bear in mind that
over the same field of view, the medium format neg is capturing
waaay more detail than the 35mm neg.
The format advantage is in the size of the negative; not in the glass
So ... compare an rb 127mm shot to a ~60mm shot on 35mm. Take a shot of something fairly flat so that DOF assuredly isn't an issue... artwork on a wall or something. Shoot the rb lens at f/8 and the 35mm lens at f/8 or f/5.6. Then print the medium format and the 35mm neg to 12" or whatever. Then we'll talk