BOAC:
'Run-back' ice is an age-old caution on all types I have flown with bleed de-icing and is, in my opinion, valid.
Never heard of it with respect to "hot" wings. Certainly have, though, with boots. My impression was that "hot" wings converted the ice to water, which did not reform as ice further back on the wing.
My former employer spend a lot of ground school time explaining the policy and critical nature of engine anti-ice. They spent very little time, however, on wing heat other than when not to use it (and reference to the OEM's AFM statements on use of wing heat). Nothing was ever mentioned about run-back ice. I don't believe I've heard of run-back ice stated as an issue with "hot wing" aircraft before this thread.
Ground deicing was a different matter, of course.