T+00:16 - Shortly after liftoff, when the engine graphic first appears, three engines were out. https://youtu.be/-1wcilQ58hI?t=2721

T+00:28 - Sparks are seen in the engine exhaust and some flashes. https://youtu.be/-1wcilQ58hI?t=2733

T+00:40 - A fourth engine is marked out. https://youtu.be/-1wcilQ58hI

T+01:02 - A fifth engine is marked out (just after the announcer says "we're throttled down and throttled back up") https://youtu.be/-1wcilQ58hI?t=2766

T+01:08 - More sparks (streaks) come from the engine https://youtu.be/-1wcilQ58hI?t=2773

T+01:16 - There is a shot of the aft of the rocket and it looks like six engines are out https://youtu.be/-1wcilQ58hI?t=2781

T+01:19 - MaxQ is called https://youtu.be/-1wcilQ58hI?t=2783

My totally uninformed speculation is that the first stage did not achieve the target velocity (and trajectory?) due to the 6(?) engine failures so the first stage continued to burn and the second stage never separated because the separation conditions were not met (velocity/trajectory).