@jkalias, thank you for your prompt response.

While I may not have a concrete test case to demonstrate the issue, it's essential to highlight that the standard Levenberg-Marquardt (LEVM) stopping criterion is ||ΔP|| <= xTol * (||P|| + xTol). Please refer to the stopping criteria as defined on page 26 of Madsen 2004

The original buggy code, using P.DotProduct(P), unintentionally produces an early stopping effect. For instance, let's dig into the details of the BoxBod_LM_Der() unit test, specifically examining the section where the condition parameters < upper, scales is applied.