This exact type of failure isn't at all rare. It's caused by unmatched flex properties between the post and frame.

Makers like to claim that it broke because the post didn't extend deep enough (ideal is 1.5+ diameters below the top tube). If so, the seat tube mast above the weld is taking the full load, and your weight pushing the post backward eventually fatigues and breaks the tube at the weld. If this is actually the case, then it might be your fault.

But it gets interesting, because the identical failure can happen even if the post extends well below the top tube. The issue is that the post can be more flexible than the seat tube mast. In that case, the flexing post doesn't contribute it's predicted share to the structure, and the mast is overloaded almost as much as if the post were too short. The phenomenon of ineffective load sharing is well documented in the field of seamanship, where boats break loose because lines fail totally, with those of the stiffest construction failing first, and others following in sequence, until the boat is completely free of it's moorings.