Hi, Pawel-
Well, after a looong day yesterday of building AFNI versions, testing, and investigating code.... you were absolutely right. A bug had crept into 3dSkullStrip via a side function. Thanks very much for bringing this to our attention (with a good example dset to test on).
We have fixed the bug, and started a build about an hour ago. So, in another hour or two, the updated binaries should be downloadable with "@update.afni.binaries -d". The version you will want to end up with is at least: AFNI_21.0.19.
thanks,
pt