W_FCA_DITHER not set in arcs

I just tried to extract a trace from an arc (opinions are divided as to whether this is a good idea, but that's beside the point), and got:

constructFiberTrace WARN: Keyword W_FCA_DITHER not found in metadata; ignoring flat for {'visit': 8937, 'dateObs': '2017-12-20', 'site': 'L', 'category': 'A', 'arm': 'r', 'spectrograph': 1}

W_FCA_DITHER should be set for all exposures, but apparently is only set for flats.