BOLD activity showing up in CSF?

Hi, I have collected fMRI data from 2 groups; preprocessed with fmriprep 20.2.0 and conducted first and higher level analyses with FSL v5 (participants from both groups had minimal motion, no participants with >20% volumes scrubbed). When comparing groups on one of the contrasts, there appears to be significantly greater activation within the ventricles, and I can’t figure out why. Does anyone know why this might be and how to troubleshoot?