In both videos after combustor ignition the turbine wheel begins to pulse for some reason. After a few moments the pulsing stops and the turbine wheel glow becomes stable. A few moments later the turbine glow disappears despite the engine still running.
It's explained in that video ->
Engine uses "auto fuel enrichment" during start causing flashing until start is complete.
The Garretts pulse in more fuel to accelerate the engine faster during start. with doing that they'd either hang or take about two minutes to start.