How to make AddSoftSameVehicleConstraint() trigger penalty when one node is dropped?

[zeldaxlove64]

With
routing.AddSoftSameVehicleConstraint([index1, index2], penalty)
I noticed that if one of the nodes is dropped (i.e., not assigned to any vehicle), the penalty does not get applied.
In other words:

• The penalty works when both nodes are assigned to different vehicles.
• But if one of them is dropped, the penalty is skipped.

What I want to achieve is:
The penalty should also apply when the two nodes are not on the same vehicle, including when one or two of them is dropped.

Is there any built-in way or parameter to make AddSoftSameVehicleConstraint behave like that?
Or do I need to manually add a custom constraint using solver().Add() to model this logic?
Any hints or recommended approaches would be greatly appreciated.

Supplementary Notes:
From the perspective of real business requirements, in my use case, there may be LAYOVER or NEARBY relationships between tasks. Both of these relationships tend to encourage assigning the related tasks to the same person. Each relationship has its own WEIGHT, which affects the penalty value.

• If there is no time conflict between the two tasks, I use AddSoftSameVehicleConstraint().
• If there is a time conflict, a new merged task is generated to represent performing both tasks together — it uses the earliest start time and the latest end time, and is set to be mutually exclusive with the two original tasks. The weight is then added to the drop penalty.

For example:
DROP_WEIGHT=100000
Task-A LAYOVER with Task-B, no time conflict, LAYOVER_WEIGHT=1000
Task-B NEARBY with Task-C, time conflict, merge to Task-BC, NEARBY_WEIGHT=10
Task-C LAYOVER with Task-D, no time conflict, LAYOVER_WEIGHT=1000

Then I would construct the following constraints:
routing.AddSoftSameVehicleConstraint([Task-A, Task-B], 1000)
routing.AddSoftSameVehicleConstraint([Task-C, Task-D], 1000)
routing.AddDisjunction([Task-B, Task-BC], 0, 1)
routing.AddDisjunction([Task-C, Task-BC], 0, 1)
routing.AddDisjunction([Task-A], 100000)
routing.AddDisjunction([Task-B], 100000)
routing.AddDisjunction([Task-C], 100000)
routing.AddDisjunction([Task-D], 100000)
routing.AddDisjunction([Task-BC], 100000*2+10)

Ideally, Task-A and Task-B should be assigned to the same person, Task-C and Task-D should be assigned to the same person, Task-BC should be dropped.
But because the penalty of AddSoftSameVehicleConstraint() will be skipped when task is dropped, selecting Task-BC and drop Task-B、Task-C causes the two LAYOVER_WEIGHT penalties to be excluded from calculation. So the final solution will choose Task-A, Task-BC, Task-D.