When the Data Dictionary Manual Override feature is set to a desired behavior e.g. Force Read Only or is Required for a specific field that appears in an External Client portal, the override may not take effect in the Landing Experience.
Things to check
Navigate to the Data Dictionary Field Override Details screen for the field in question.
Page Id - this should be blank
User Type - select <External Client>
Note that if the Page Id is populated, the data dictionary manual override is honored in the HR / Business external client portal but not for the Landing Experience. For it to be honored in both portals, save the data dictionary manual override record without any value in the Page Id field. Remember to press [Clear Cache] for the changes to take effect in the portal.