This is explicitly written in ExtJS documentation :

** This process will cause setValue to asynchronously process. ** This will only be attempted
 *    once. Any unknown values that the server does not return records for will be removed.

As consequence, we cannot use a same Tag field when we need to do #setValue more than once.

Sample of use case :

• Open a dialog box which contains a Group field
• Init the group field by #setValue("group-1")
• The store of this field is not yet loaded, so "group-1" is not in the store (unknown value)
• The store is loaded to ask "group-1" to server side, then #setValue is re-called into callback (OK)
• Close the dialog (the form fields are not destroyed but hidden)
• Re-open dialog box
• Init the group field by #setValue("group-2")
• "group-2" is a unknown value but the store is will not be loaded as it was already loaded (KO)