There are some problems with reproducing accurate thermal neutron capture gamma ray spectra for (n,gamma) reactions fir some isotopes when using the HP physics.
In short, agreement is is good for some nuclei and not for others with no obvious (to me) pattern. I’ve never checked out cobalt to see if it is one of the culprits. Playing with two of the NeutronHP physics UseOnlyPhotoEvaporation and DoNotAdjustFinalState can sometimes improve results.
See the thread Thermal neutron capture gamma spectra give major disagreement with NNDC data base for some isotope for more details.