I expected some no-name Chinese make but no , it is Neutrik with this crappy
design.
Reassembled preamp with the socket exposed and it was then obvious there is
a third way to loose ground contact. There is no sprung contact to the
sleeve of the jack , just rests on the chromed barrel , relying on the
spring contacts of the tip and ring to supply pressure to keep the sleeve in
contact with the inside and innermost small segment of arc of the chromed
barrel . Even without WD40 , corrossion and general crud , because of the
lever action, it takes very little upward force on the jack, at the cable
end , to lift the sleeve and break contact.

So failure mechanism probably went like this.
Bad ground contact due to slack nut. Instead of tightening, owner squirts
WD40 in there which
pools in the lower surface of the barrel , collects crud, congeals etc.
Eventually he tightens the nut , but by then its too late. A get by , on the
road, solution to this would probably be some sort of reaming action inside
the bush barrel