Just watched the test again and here is more wood to add to the fire.
The clock in the top right corner of the facility cameras starts counting at engine ignition command and stops at 67.7 seconds. This puts the cutoff +/- at T+60s (judging from the H2 burnoff igniters starting at T-12s and also engines going to 109% at T0 right before the camera view changes).
A TVC issue is still on the table, but given how close the cutoff is to the engines throttling down to 95% (right before the TVC test), an engine issue cannot be discarded. The first thing that jumps to mind is an hydraulic lockup in an engine. This would not be a problem during steady state operation, but for throttling it needs hyd pressure to move the valves, so at that point the controller would go into "hydraulic lockup mode". This doesn't shutdown the engine by itself (it happened on STS-3), but as this a test, and in the interest of safety, it is possible that it would mean the end of it. This fits with what was said yesterday, that "an engine sent a signal to the core to shutdown".
Interesting how it all comes back to hydraulics...
After shutdown the test controllers are heard saying that all 4 engines are in "post-shutdown standby" (the regular post firing controller mode) so they obviously have telemetry from the engine controllers, thus discarding a full blackout of power in them or a data path issue. A pneumatic shutdown (which is how the engines are killed without hyd pressure) also ends up in post-shutdown standby, so no conclusions on this can be taken.
Post firing, it is visible that the white covers at the top of the nozzles are ripped, especially in the area between the engines.