overwrite fails for certain fpacked images

[beckermr]

An fpacked image with keywords is failing to allow me to overwrite it. The example image is at BNL here:

/gpfs/mnt/gpfs02/astro/workarea/beckermr/des-y6-analysis/2022_02_04_imsims_tests_desy6/sim_outputs/des-pizza-slices-y6-v12/DES2359-6539/sources-r/OPS/finalcut/Y5A1/r3515/20170821/D00670401/p01/red/bkg/D00670401_r_c56_r3515p01_bkg.fits.fz

I see the same mean in the image before and after, so this appears to be noop.

@esheldon verified that this does work for a simple example script so we think it has to do with settings for this specific image.