You will need to republish the feature service in order for the new field to be visible in your feature service. At least in the old Portal map viewer, any new fields you add are hidden by default in the pop-up, so you'll need to make sure that the new field is turned on in the pop-up after republishing the feature service.
And just to clarify, by republish, I mean you can republish by overwriting the existing feature service. You don't have to create a new one.