Losing image mask if it has a Link behavior applied

Is anyone else seeing the following?

1. I have a group of shapes, I use the group as an image mask on layer A.

2. I apply a link behavior to the group (or to one of the shapes in the group) so that its position is linked to another layer B.

3. All works well.

4. I save the document - suddenly the image mask source on layer A is empty

5. I can no longer use the group as source for the image mask on layer A - the mask source well doesn't accept the group.

6. If I delete the link behaviors in the group, the layer A mask source well accepts the group.

rinse and repeat...

If you've had the problem have you discovered any workarounds?

Alex