I presume you're encountering this at or near infinity focus setting, when the optics will be closest to the film? Does the lens in fact focus to infinity, or does it go "past" infinity a bit, suggesting that the Yashica it was originally made to fit might have used a subtly different mount (same threads, different reference distance)? I seem to recall the Yashica SLRs as being M42, though.

The other possibility is indeed that the Yashica SLR might have had a smaller mirror; I'd guess you only have a millimeter or so of actual interference at closest setting, and since few SLRs have 100% ground glass, Yashica might well have accepted a millimeter of vignette on image top (mirror bottom) in order to clear a lens with more protrusion.

Still another possibility -- either your Auto Yashinon or your Spotmatic SP is a hair out of adjustment. But I'd guess the first place to look would be the infinity stop in the lens.