I did not test this on the branch I commit from, but I used this code:

def get_path(self):
 """Return the mutated path of the rectangle."""
 boxstyle = self.get_boxstyle()
 m_aspect = math.sqrt(self.get_mutation_aspect())
 # Call boxstyle with y, height squeezed by aspect_ratio.
 path = boxstyle(
 self._x * m_aspect,
 self._y / m_aspect,
 self._width * m_aspect,
 self._height / m_aspect,
 self.get_mutation_scale(),
 )
 return Path(
 path.vertices * [1 / m_aspect, m_aspect],
 path.codes,
 )
FancyBboxPatch.get_path = get_path

And on top of the code in the linked issue and the resulting plot is the second plot on that issue.

As discussed in the issue #28876, the behavior is consistent, reasonably documented (after #28900) and suits common usage patterns. Therefore, we don't want to introduce a breaking change through changing the behavior.

@Balthasar-eu anyway thanks for your contribution. While your PR did not make it, it lead to an improvement in our docs. 🎉