Hello ,
As per you , you are able make CLI changes but the IPV6 tab in SRX Web GUI is inactive . Either its that the IPV6 is not enaled : https://kb.juniper.net/InfoCenter/index?page=content&id=KB25697
Or its an issue with J-Web . So inorder to identify this I asked for the output . If its J-Web issue , we may need to open a J-TAC ticket to see if there is any fixed version .