Why is the Select Record form missing some database fields?
When you click on "Select Records", (or do a Print Preview), by default, it will only show the columns for the fields used on the label. When bringing it up with no fields in use, all columns are shown. When you add a text object to the label and assign a field to it, there is now one field in use so showing the select records form will now show only the one column.
However, you can modify such default behaviour in BarTender. Within the Select Record dialogue, the right-click context menu contains additional options that allow you to full customize which columns to show in the form.