I am experiencing the same issue as described above.
Re-running fmriprep with --force-bbr did not fix the N4 bias field correction. Outside of fmriprep, I called
N4BiasFieldCorrection with the fmriprep arguments with the addition of --shrink-factor 3. That manages to produce a reasonable image.Is it possible to run the N4BiasFieldCorrection inside the fmriprep call with the
--shrink-factor 3 included?