This is certainly a bug. What happens now is that when a mixin with an inline mixin is found, there is an error in admin, and it stops processing the rest of the parts. So it is kind of random which parts you will have available in that case.
We need to check if mixins within mixins can be supported, and if not show a clear error message.
It should also be made more robust. Errors in a mixin must not affect other components that are not using it.