exhaust leak at the gasket

Anyone got a sense of why these flange gaskets blow in the first place? I found damage on the end of the manifold pipe that wasn't filed down by muffler shop and flange gasket they put on blew out in less than 300 miles. I replaced it with new gasket (see pic) after prep to the damaged pipe / ground down enough for the new gasket to fit.

It's a very difficult space to reach way up under the car, but with a handheld grinding stone and sandpaper and my hand getting exhausted, I got it done.

Anyways, the engine burning lots of oil ended and became just a bad valve cover gasket leaking oil on exhaust manifold and running rough with no warning lights. But those are just symptoms. What's the cause of this?

Exhaust pipe pressure is normal and valve cover air pressure with oil fill cap off is normal and no error codes, but once the engine warms up it runs a bit noisy and oil starts frying on exhaust manifold grill and it starts sounding like this cars exhaust manifold flange gasket is again about to blow.

So I'm going to replace the valve cover gasket and PCV valve this weekend. Already did throttle body and spark and MAF sensor cleaning, new 12v, reconditioned Hybrid battery, etc.

If there's anything I'm overlooking, please let me know?

Here's more detail: First Timer with an issue | PriusChat