So we found out the reason why IFT-2 and IFT-3 boosters exploded and IFT-3 ship lost attitude control. The reason is autogenous pressurization. Typical autogenous pressurization is done with cryogenic propellants being heated up in some form of heat exchanger inside the engine/nozzle and then being fed back to the main tanks.
https://en.wikipedia.org/wiki/RS-25#/media/File:Ssme_schematic_(updated).svg.svg)
https://www.reddit.com/r/spacex/comments/cxkrtb/detailed_diagram_of_the_raptor_engine_er26_gimbal/
However, Musk just told us that they deleted heat exchanger and used pre-burned gas instead.
The difference is that instead of pure oxygen or methane "oxygen (or methane) rich gas" is being introduced into the tanks. It means that it is not pure component - the component was already pre-burned in the pre-burner and contains combustion products, mainly - H2O and CO2. Those tend to solidify in the cryogenic tanks and cause all sorts of issues, including filter blockages, valve blockages, engine explosions, loss of attitude control etc.
To be honest, I'm sort of shocked that SpaceX uses this solution. Was this approach ever used in a rocket engine before? Is this a consequence of Musk's doctrine "the best part is no part"?