I'm wondering if fuel ran out right before impact. It looked like only two engines ignited for descent (I assume one had failed, like it appeared during ascent?), and engine #2 flamed out a bit before impact, followed by the last engine flaming out just a few seconds later, ~1 second before impact.
That was an excellent video analyzing the entire flight! So from that analysis, there were no engine problems during ascent, and the two engines being shut down near apogee was intentional. So the only major problem was the low fuel pressure after the flip back to vertical and reigniting two of the engines (the third engine was not intended to restart for landing): due to low fuel pressure, engine #2 flamed out, and engine #1 started destroying itself due to running lean (too much oxygen, not enough methane) and thus burning the copper internals, resulting in green flames and low thrust.