There is always one unassigned node when setAmortizedCostFactorOfAllVehicles and addDisjunction are used together and the problem has one vehicle

[KeremAslan]

What version of OR-Tools and what language are you using?
Version:
JVM: v8.1.8487
Python: v8.2.8710
Language: Kotlin/Python

Which solver are you using (e.g. CP-SAT, Routing Solver, GLOP, BOP, Gurobi)
Routing Solver

What operating system (Linux, Windows, ...) and version?
MacOS

What did you do?
When using the function setAmortizedCostFactorOfAllVehicles in conjunction with addDisjunction, I have found that there is always one node that gets unassigned. I use Kotlin in my project, but I have set up a minimal reproducible example in Python that also has the bug. Please find the project here.

What did you expect to see
In my create_simple_model() function I have one pick-up node and one delivery node and I expect both nodes to get assigned as the penalty on unassigning them is very large. Instead, OR-Tools always unassigns one node so long there is only one vehicle. The moment you add a second vehicle, it is able to allocate both nodes to a single vehicle and the solution returned is as expected.

The fact that I can reproduce this using your example makes me think there is a bug in OR-Tools where setAmortizedCostFactorOfAllVehicles doesn't work well when the problem has only one vehicle. Can you help check what that bug could be? Many thanks!