rank on customer item number not respected

Summary:

My client uses customer item numbers intensively and all works well. I can enter orders using internal item numbers or customer item numbers. The customer item number is also stamped on the order line.

But when item/customer combination has 2 entries that have a rank 1 and rank 2, the behavior is unexpected!

Client says that in their current legacy system (Oracle eBS) they do not have this issue.

Content (please ensure you mask any confidential information):

The unexpected behavior is that when I enter internal item number, I get a warning FOM-4515483 and it implies I need to use the search and select one intance, or continue but then the customer item number is ignored.