Wasn't there some plumes that remained aloft from that part of the world recently? Not all volcanoes are as well measured
Mount Sinabung in Sumatra seems suspect: "On 07 March 2014, an eruption resulted in an ash plume extending west and reaching a height of 4 km"
Thoug if the aircraft flew through one of thsoe and it caused the engines to fail there would still be time to put out a mayday call.
There was an incident with a British Airways 747 back in 1980 or thereabouts when volcanic ash caused all four engines to shutdown but they were able to get them restarted.