Why do link constraints not work?

I have a DNG project area set up with Link Constraints and different artifact types (A and B).

My link constraints allow links between A and A also between A and B

The problem seems to be ... If I create a copy of an artefact of type A, and change the type of the copy to type B then I cannot create a link between the original and the copy. However, if I don't change the type of the copy, I can create a link and then change the type later. So ... What's going on?

Does anyone have any ideas what I'm doing wrong (I know I could use duplicate and link but there are reasons why I don't)?

TIA

Mark