The same happens to me.
Please I need help!, I followed the tutorial http://nemops.com/new-customer-address-fields-prestashop/#.Vg6ILPl_Okr to add three fields.
With five steps checkout everything works fine, but on one page checkout I get error saving the fields, field x required,
like there were empty, like there were not in the model.
Also in ObjectModel address :: $ definition if I put the required data is saved, otherwise not, no matter what the type of checkout.
Anyone can help me?