Is there a bug with Enumaration Lists stored on database?

I created an Enumeration List stored in the Database and an attribute based on that Enumeration List.

When putting a value in that field, it appears a message “+Add xx to value set”, which is correct the first time that a new value is used. But after any value is added to the list, when I trying use that value to filter the list, it continue appearing the same message, despite the value was already added to the list.

Is this a bug or I misunderstanding this kind of attribute?

I found the same behavior in 6.0.2 and 7.0.2 versions (environment: Windows Server 2012, DB2, WAS)