When the freezing point of fuel is measured on the ground is the resulting figure corrected for the increased pressure found in the fuel system?
I think the freezing point of fuel (but not water) is raised by increasing pressure. In which case you might speculate on the possibility of having liquid fuel in the tank... which then freezes in the pipes/pumps when pressurised.
This effect probably isn't great enough though.... -34C to -57C is a bit of a stretch and the architecture of the 777 fuel system may also rule this out.
Edit: Initially when you compress something heat is given out which raises the temperature. At high flow rates the fuel might remain warmed. At low flow rates perhaps this heat has time to disipate leaving the higher freezing point to cause the problem. If that's correct it might explain why this didn't happen in cruise just on approach.