SN10 (Starship) High-Altitude Flight Test
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At this point I'm quite certain they're going to iron out the kinks and will be able to get this consistently right soon enough.
[1] Starship basically does a belly flop to slow down and only flips to an upright position just before landing.
So... what's burning!?
What you are referring to however is most likely related to access gas. Starship starts up 3 engines, and then shuts down two of them to land with 1 engine.
Both during the startup and but most shutdown, some amount of access methane escape from the engine. Since the other engine is still burning and hot, this access gas can start to burn. This is not all that concerning most of the time as long as there is not a continual leak.
Every kilogram matters, because to lift that kilogram you need even more kilograms of fuel, but to lift that fuel you also need some more fuel! It's brutal.
So you save and you cut and you leave out everything that isn't absolutely necessary. Air resistance is almost free. You do need control surfaces, but they're small and hollow flaps. Fuel is heavy.
That's why 100% of spaceships use aerobraking during reentry. It's much, much cheaper than any kind of sci-fi powered descent.
It turns out that even doing the flip slightly higher, already has a massive impact in terms of energy required.
Slowing down from Orbital speed is an insane amount of energy that has to be removed. You really only want the last tiny bit to be removed by rocket thrusting.