This is only relevant for Gurobi 9.0 and earlier versions. This issue has been fixed in Gurobi 9.1.
Sometimes the Runtime attribute incorrectly returns 0 after optimizing. This is due to a known issue with calling model.update() or model.write() before querying the Runtime attribute.
To obtain the correct runtime, please query the Runtime attribute immediately after the model.optimize() call. This incorrect behavior will be fixed in the next major release.