It's really down to the GNC, but normally we'd expect the bad engine to be shut down, and the burn to continue on the single engine for longer. With modern guidance software you wouldn't expect it to just count the duration and stop.
Some possibilities I can think of are:
* Failure of 1st engine also affected second engine, perhaps a violent failure that damaged the engine or prop lines.
* Attitude control was lost on the stage, meaning no way to complete the burn.
* 1st engine failure damaged prop tanks leading to fuel depletion.
* Underperforming engine lead to fuel depletion: Bad engine was consuming propellent at regular rate and generating almost no thrust. This feels unlikely since the expander cycle engine needs the combustion heat to run the pumps.
* Blue Origin played it safe and wanted to ensure a safe de-orbit, so as soon as it saw engine problems it cut loose the payload and deorbited GS2 on the single good engine.
