When using the (optional) convergence criteria for the optimisation loop, adherence to them is checked in the postUpdate phase of the update of the design variables. If a line search is used, this means that the flow equations have been solved anew using the new design variables and the objective reduction check makes sense. If, however, no line search is used, the objective reduction check happens immediately after the update of the design variables and before re-solving the flow equations, giving a reduction in the objective function and terminating the optimisation loop prematurely.
Solved by calling the convergence check directly after the solution of the flow equations, in case of a fixedStepUpdate