Don’t think a MANPAD can hit 31k feet. Probably a Buk again. Or something similar.
A British C-130 was downed in Iraq in ‘06 flying at 30,000’. The insurgents, AKA Al Qaeda-in-Iraq, AKA ISIS video taped it and I saw the video. They lined up under the known flight path between 2 navigation beacons and fired unguided rockets at approaching aircraft. Eventually they hit the Brit C-130. It was blown to pieces with wreckage scattered all over.
The solution was to fly slightly offset the flight line rather than directly between 2 points. Never happened again.