Good point!If testing a ring final circuit for end to end continuity, then even if rn is open circuit, a plug in socket tester will not detect this as it will still "see" one of the two neutral conductors.
Unless there is more than one break in the continuity of rn. (Same applies of course to each of r1 and r2)
I had been assuming it was a non-functioning socket going undetected and not an RFC break.