Height doesn't matter. It's about bending moments and the location of the fulcrum.

Imagine inserting a strong but flexy rod into the frame and pulling it back the way a post would be under load. Would the rod flex ONLY above where it exits the tube? Or, would it tend to pull the tube back with it?

Repeat this countless times to imagine how the design combined with in sufficiently an rigid post would cause a fatigue failure.

You can also search "aluminum edge crack formation" or a similar phrase to learn why aluminum frames are especially likely to fail this way.

FWIW a taller extension might do better because the slot and drilled hole would be farther from the most stressed area.

Flex is the issue, and eliminating it is the solution.