Please follow the below steps to resolve this issue.
- Go to Modeling language page
- Click on Manage profiles of particular MDG
- Check whether the custom attributes are added and not hidden in the Form designer page. If not, add attributes by drag and drop and enable eye icon
- Click on Save in the Form Designer and Update Profile in the Edit Profile page.
- Set the profile in which custom attribute is added as Active
- Go back to the properties window and check