Why does not the red mark disapeear in a script based value set attribute?

I use script based value set to customize an attribute in work item. Scripts set up a dynamic value set in this attribute sucessfully. At this time, this attribute has a red mark as the default value is not a valid one. Aftter user selects one valid value from drop-down list, this red mark is gone. But user click Save button, saving is completed without error, but this red mark comes back again. You have to refresh this work item, this red mark will disappear.

Did anyone encounter the same problem and resolved this issue? Please share your experience ASAP, thanks a lot, lot...

Jeff