OK so having examined the spectrum images it appears that the uptick in the spectra at the red edge is due to an additional component superimposed over the correct spectrum. This is seen as faint region at the top of the spectrum seen as a lump in the cross sections to the right of the unchanged main cross section profile. See also the attached image.
The origin is unknown. (suggestions welcome) but a working hypothesis (to be tested) is a second order overlap but that is surprising to me at least at this wavelength