When im put Zephyr im 2 way travel in one location they load some material and a other city unload and load different RSS they after some time it starts loading materials into other slots.
For example,
1 way - bear fur slots 1 and 2, seal fur slots 3 and 4
2 way - slot 1 and 2 parks, slot 3 and 4 sleeping bags,
After some time we have seal fur also on slots 1 and 2 which causes the ship to be clogged with materials that it does not discharge.