Binding to sub-property of attached property fails silently or something

[Splitwirez]

So, for example,

{Binding Path=(mecore:CornerCurves.CornerCurves).TopLeft, RelativeSource={RelativeSource Mode=TemplatedParent}}

fails silently, when it should work (and does in WPF).

Real-world example can be found here: https://github.com/Splitwirez/Mechanism-for-Avalonia