The report also mentions a planned descent to FL315 due to "extreme cold"....which did not happen because of an ATC request.
Also mentioned was that later in the flight at FL380 over the Ural mountians the oat was -76. The met office decscribed this as "unusually low compared to the average, but not exceptional"
They seem to be considering the possibility of fuel icing, however the fuel never got below -37.
Puzzling indeed.