In the BOM of an engineering change order (ECO): 

• I get an error message when I add a new circular reference, i.e. when I create a BOM line to add another version of the same item number. 
• However, I manage to modify an existing circular reference (created from the BOM of the engineering version), i.e. to modify the version of this same item number on the BOM line. 

In my opinion, this should be solved because it should be possible to create a second engineering version via ECO whose BOM contains the first engineering version of this same product (since in this case there will not yet be a circular reference to modify).