ConstraintReadOnly Property Behaviour with Cfg_OnConflict Script

Dear Experts,

Can some one clarify the below thing is expected behaviour or Bug .

Test Root Product       (Attribute ->  ABC  , freeform , with ConstraintReadOnly =Y Property)

   Test Component A

   Test Component B

   Test Component C

Rule : When the attribute ABC = C excludes selection of Test Component C from TEST OPTIONS.

Case 1:

No Script in Cfg_OnConflict.

step1: Enter "C" Value in Attribue ABC.

Step2: Test Component C will be Excluded.

Step3: When Try to select Test Component C Proceed Button is Disbaled. - Due to Constraint Readonly.  Its expected behaviour.

Case 2:

Put below script on Root.

function Cfg_OnConflict (Explanation, Resolution)