I don’t fully agree with your statement regarding the intention of the original design. The FOHE is used to heat the fuel for ice protection. The FOHE forms part of the engine Thermal or Heat Management system, which to quote the Rolls Royce engine book “[I]A responsibility of the heat management system is to ensure that the fuel temperature is above 0 deg C at the critical and vulnerable parts of the fuel system for the majority of operating conditions. The heat rejected by the engine oil system via the FOHE is normally sufficient to ensure fuel temperature above 0 Deg C, so protection the fuel system from fuel-borne ice.”
I recall that Boeing has made it’s own investigation on the issue (white paper) and seem to believe that the reason that the other B777 engine (PW) has not suffered similar events is due to the engine fuel system architecture; warm spill flow from the FMU is re-introduced upstream of the FOHE and therefore warms the colder fuel coming from the aircraft fuel tanks, before it contacts the face of the FOHE.
It must be noted that the RR Trent 800 does not re-introduce warm spill flow upstream.
I wonder if a further modification for future RR engines will change this philosophy instead of removing tapered ends of the FOHE tubes…….