The CP-SAT solution in VRP Routing Solver is not returned

[chenyw1994]

What version of OR-Tools and what language are you using?
Version: v9.11
Language: Python

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

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

What did you do?
Steps to reproduce the behavior:

1. I builded a very simple vrp model by using pywrapcp.RoutingModel()
2. Set search_parameters.use_cp_sat = BOOL_TRUE and search_parameters.use_cp = BOOL_TRUE to force the solver to use the CP-SAT
3. I call "solution = routing.SolveWithParameters(search_parameters)"
4. CP-SAT get the optimal solution. I can see the logs: CpSolverResponse summary: \n status: OPTIMAL
5. But the result of CP-SAT not in 'solution' and solution.status() == 3 which means routing fail.

What did you expect to see
I can get the CP-SAT results.

Make sure you include information that can help us debug (full error message, model Proto).

Anything else we should know about your project / environment