I am not sure I follow this. I can understand how the wavelength dependency of the Airy disc size due to diffraction could potentially cause this effect in a diffraction limited situation but in your setup I would expect this to small compared with the slit width and the seeing. Also the atmospheric effects which affect seeing and atmospheric dispersion are due to refraction, so wouldn’t they reduce with increasing wavelength? Comparing the star FWHM from photometric B and I band measurements could be interesting. Do you know if that this affect on seeing is seen there ?