Before the VariableSets on the Connections are set correctly, they are already used by calling "ProjectionOptimizationHelper.SetupSystemAccessOptimizers" (in ProjectorEngine.ExecuteAsync()). Problems are caused when a variable from the DefaultVarSet throws an exception via script when the DefaultVariablenSet is used. But for sync you should use a special VarSet.
This is a product defect (31196).
This will be fixed on a future release of the product. If you require this immediately corrected, please contact support for a hotfix referencing the defect ID 31196.