Sorry, not able to give too much in the way of details. However, generically -
(a) aircraft has a small section of the fuel system which is susceptible (ie somewhat exposed) to OAT variations
(b) longish flight at high level
(c) chilly air mass
(d) waxing caused a reduced fuel flow through the relevant bit and consequent downstream tank quantity alert (ie more going out from the downstream tank to the engine than coming in from upstream tanks)
(e) crew twigged that it was not likely to be a tank fuel loss problem but rather a fuel flow imbalance problem. Either way OAT was determined to be the likely culprit
(f) descended to a level where they could maintain cruise with reduced power settings until the wax dissolved (this descent, as you might suspect, was more in the nature of a programmed decision rather than a commanded action ..)
Problem - had they dilly-dallied at height trying to fault find, the waxed up region might well have gummed up totally and caused total fuel starvation to both moteurs with heart palpitations the likely result as they were over a watery area and a long ways from land.