The intensity drops corresponding to transitions between magnetic hyperfine levels are smaller at 50K than at 18K. So it would look like the magnetic hyperfine interaction is lower at 18K (less splitting, so more resonant photons not measured by detector). I think this is the case, if the temperature is lower, there is less uncertainty on the size of the nucleus, its protons and neutrons would be more stable (less thermal motions), which would make the nucleus more spherical so that its quadrupole moment would go down, so the quadrupole interaction would be smaller, so there would be less splitting of the 3/2 level. This would result in a deeper peak at 18K.
I also believe the peak in the middle would be deeper at 50K than at 18K because the mean square radius of the nucleus is smaller at lower temperature, so this energy shift would be smaller. Again because the protons and neutrons in the neutron have less thermal motion. I think these rout mean square radii becoming small for both levels would make the energy difference smaller which would give more resonant scattering.