The large storage unit in my screenshot is supposed to be set to allow export. Trucks are only allowed to pick up iron plates from the large storage unit, which happens only when the small storage unit (which does not allow import or export) backs up into the large storage unit.
The large storage unit keeps resetting the export permission to disallow export, which is the default behavior when chaining storage units. This happened at least when I upgraded it from a small storage unit, and again when I upgraded the belt that feeds the large storage unit.
I have also seen this behavior with the loose storage units.
