PartialDerivativesEquations does not work with non-Cartesian elements
Using PDE with the propagator's orbitType set to a non-Cartesian value does not produce meaningful results. See the discussion on the dev list: https://www.orekit.org/wws/arc/orekit-developers/2017-08/msg00010.html
One workaround is to compute the STM in Cartesian elements and then transform it to the desired element set using equation 7.32 in Montenbruck & Gill:
STM(Keplerian) = d(kep)/d(cart) * STM(Cartesian) * ( d(kep)/d(cart) )^-1
Another workaround is to use a FieldNumericalPropagator with the orbitType set to the desired value, and then extract the STM from the final orbital elements.