From above figure and the explanaition given here 

I expected, that the following should work and validate without errors:

A Block "Example 1" composed of three parts A, B, C each with one ProxyPort typed by IF_A, IF_B, IF_C where the ProxyPorts of A and B connect to the single proxy ProxyPort of C: 

This apparently works and does not give any validation errors. This is expected because compatibility should be by feature. If I add another FlowProperty to any of the InterfaceBlocks that remains "unconnected", I get a validation error as expected:

However, if I add another ProxyPort typed by IF_Ext to the parent block, extend IF_C with the additional FlowProperty and connect to C like this:

 I get a validation errors "Flow is not possible! No compativle flow properties provided".

Why is that? Do I misunderstand the SysML Spec or is this an error in the validation rule?