We need to know more. Several scenarios suggest themselves, but we don't know how you've related your tables and exactly what options where enabled for the relationships and for the portals. I can see you've enabled portal sorting, but there could be other factors here.
When you imported the data, was there a creation date to import? Did you enable auto-creation during the import? Any chance that a modification date instead of a creation date might be specified for the date field?
Do the two portals refer to the same table or different tables? (Clearly you have something different between the two to see the difference in behavior, but this could be in the data or it could be in what you've specified for the portal and/or the relationship involved.)