This usually happens when your devices aren’t all pulling from the same contacts source, even though iCloud has the correct list. On your iPhone and iPad, go to Contacts settings and make sure iCloud is set as the default account and syncing is turned on, then refresh or toggle it off/on. In Microsoft Outlook, check that the iCloud contacts folder is selected and not a local or Exchange list, since the “To” field only shows contacts from the active address book.