Resolved in Gurobi v9.1.1: Pre-specified user cuts

Affected versions: Gurobi v9.1.0
Fixed in version: Gurobi v9.1.1

Issue

In Gurobi 9.1.0, there is an issue with the handling of pre-specified user cuts that are >= inequalities.

These inequalities are mistakenly treated as <= inequalities, which can lead to wrong answers.

Note: A pre-specified user cut is a constraint that has the attribute setting Lazy=-1.

Workaround

Specify the cuts as <= inequalities by multiplying the constraint by -1.