Engine relight is one of the hardest part of what they do with those Merlins. It’s one of the most incredible things that have come out of rocketry in the last few decades. Deep cycling rocket engines is a miraculous feat of engineering considering is the focused application of an explosion in one direction.
That said, note the engines that didn’t relight were on one side. It was asymmetrical which lends credibility to the unequal pressure theory. At those speeds and in low gravity the fuel was sloshing around and likely didn’t make it where it should’ve been for the relight. Time will tell.
Standing up in Earth gravity, the fuel goes where we want it. While thrusting, acceleration keeps things flowing in the right direction.
Things get much more complicated when you try to relight things in zero G.