I have a Mine Control Tower with two export routes: an Ore Sorting Plant and a Loose Storage (Limestone). However, mine trucks full of sand are delivering it to a sand storage. (The sand storage is right to accept delivery, but the mine tower shouldn't be able to deliver there, since it has export routes set.)
Here's the kicker: when I remove the limestone storage export route, trucks stop delivering to the sand storage, and deliver the sand to the sorting plant as expected. When I re-add the export route to the limestone storage, the trucks resume delivering to the sand storage.
v0.6.4c (b341)
Files not imported: mine-export-bug.save