I have encountered this before, TemplateBinding does not work for custom dependency properties on controls. See these related questions:
issues with template binding and binding of custom component
TemplateBinding does not work in certain cases(when using TranslateTransform)
I have always used this instead:
{Binding MyProperty, RelativeSource={RelativeSource TemplatedParent}}
It is semantically the same as TemplateBinding, and can also support value converters etc …