Regarding convection and why this flight had problems whilst others didn't, I feel it could have something to do with the fact that it was at a lower level (FL320) than any other flight (FL340-380).
In equatorial oceanic cumulonimbus, updraft strength can be enhanced above FL200 due to heat of sublimation of ice. Therefore it is possible that somewhere above this height and up to near FL320 the updrafts were especially strong but
also still containing some supercooled water droplets and hence icing (observations showed -29 C at FL320, but in updrafts it could have been several degrees warmer than that). Flights higher up (-35 C at FL340, -40 C at FL360) were more unlikely to encounter supercooled water, therefore they may have escaped.
So, for 8501, possible airframe/sensor icing, affecting performance as the plane climbed to higher levels, and possible some ice crystal engine shutdown then to finish the job?
I have done a detailed analysis
here.