Hamish and Magnus: Could you please post your optical trains? Jasem will likely have a better take on the data than I.
I have some spectroscopy files from my latest observing sessions in November and the DATE-OBS field is present. I don't understand the connection between plate solving and the appearance of the observation date, or how a spectrometer affects whether the field is present. INDI/Ekos isn't aware of a spectrometer.
Something I did notice: in my FITS header there is always a "Comment Generated by INDI"
FWIW, i did not see a similar line in the header from Magnus.