Here are 2 Tree Planter, both are not assigned to any Tower (I expect, they will go to any location).
If Tree Planter drives to take new Saplings (near Tower #1), then this Planter cannot find the way back to the Tower #2 area.
If I order the Planter to move there manually, then after it reaches the destination, it successfully starts to plant trees near Tower #2.
No advanced logistic control involved.
Expected behavior: Unassigned Planters will find a way to any accessible Tower by themselves.
Save game attached