Modelio 3.x, 4.x

Consider the following structure:

ClassA
|-- ClassB
|-- ClassC
|-- ClassA Class Diagram

Diagram should be empty (everything masked).

Now, add classes to the diagram in the following order: B, A, C. Notice that C won’t appear on the diagram.

Mask everything and try again: B, C, A. All classes will be added to the diagram.

Obviously, if you now tried A, B, C order, only A would be added to the diagram.

I believe Modelio forbids to add said classes because they supposedly already exist on diagram as inner elements of A. The fact that this limitation can be overcome by adding classes in other order is clearly a bug. The limitation should be either removed or enforced more strictly. I honestly don’t know what UML spec says about it.