Arcology construction supply shipments not returning
Solution 1:
It's just a SimCity bug. I fixed it by deactivating all the construction contribution flows (Metal, Alloy, and TV 'power' buttons), then deactivating all of the trade depots. After leaving all these items deactivated for 4-5 hours of in-game time, I logged out then back into SimCity. Activate the depots and the vehicles should be fixed (if not, give it some more time).