https://www.spacex.com/launches/starship-flight-12
"Following stage separation, the Super Heavy booster performed a directional flip maneuver and attempted its boostback burn. It was unable to light all planned engines and performed a partial boostback burn that ended early. Super Heavy attempted to reignite its engines for the landing burn before experiencing a hard splashdown in the Gulf of America."

Can we take it that this is adequately admitting that boostback and landing burn of booster 19 on flight 12 failed?

Previously that page had included
"The booster’s primary test objective will be executing a successful launch, ascent, stage separation, boostback burn, and landing burn at an offshore landing point in the Gulf. As this is the first flight test of a significantly redesigned vehicle, the booster will not attempt a return to the launch site for catch."

So these seem to be primary objectives for the booster on flight 12 rather than "Individual failures"

Re "Individual failures (e.g. engine failures, subsystem problems) that don't affect overall flight objections will not cause a flight to resolve NO"

I assume you mean 'objectives' rather than "objections".

Not sure if it is worth noting: Dependant multichoice so can't resolve 12 yet or others until there is fully successful attempt.

I would suggest noting whether each flight was successful or not and if not what objectives were considered relevant and failed. e.g. Flight 12 is considered to have failed due to unsuccessful booster boostback and booster landing burn [optionally, from offshore landing point: and booster simulated landing?].

This link has the page from before the flight
https://forum.nasaspaceflight.com/index.php?action=dlattach;topic=64957.0;attach=2503833;sess=54281

@ChristopherRandles Yes, Flight 12 was not successful.