Alternate forms are just different visual forms for the same Database Table. If you add a new field on an alternate form, there's no column in the table to store that value.
These are only useful for things like custom controls to add functionality without saving any data, that's why the data was not able to recovered, it never got saved because it didn't have a place to save.
You will need to either add the field to the customer class itself first (it may allow additions), or create another form that has that field and store the value there, maybe a joining table.