Zephyris opened the ticket and wrote:

(nightly-r13665 and 0.6.2 Beta 1)

Actually 2 bugs, probably related:

1. Replacement of a road vehicle with an articulated road vehicle with front vehicle of capacity zero forgets the cargo refitting of the vehicle.

2. Articulated road vehicles with a front vehicle capacity of zero cannot get replaced (the replacement vehicle can be selected but when the vehicle visits the depot the replacement does not happen) except with another articulated vehicle with capacity of zero.

If this isn't clear consider the following:
Problem 1:
Build an semi-articulated truck, ie. drivers cab (capacity zero) pulling a hopper trailer (can carry coal/grain/ore/etc.)
Refit to ore from the default cargo of coal
Auto-replace semi-articulated truck with another semi-articulated truck
On visiting the depot the vehicle is replaced, but the cargo refitting reverts to coal

Problem 2:
Build an semi-articulated truck, ie. drivers cab (capacity zero) pulling a hopper trailer (can carry coal/grain/ore/etc.)
Auto-replace semi-articulated truck with another vehicle
On visiting the depot the replacement doesnt happen

A test GRF can be downloaded from here: http://www.tt-forums.net/viewtopic.php?p=709908# p709908

Reported version: trunk
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/2146