The m-link max range is 25km, yes... however what kind of guidance law in terms of ratio of m-link/seeker lock is used ragarding launches under 25km, I do not have any information. Screenshot I posted from manual says 70% (for 27R1), but there is ambiguity, as it does not states the ranges.
However the section I also posted, where it talkes about engaging targets which use chaffs, there are no problems for guidance. So if you keep lock, chaff makes no difference.