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This is not surprising given that you don’t have the data required to perform that particular step in the correction:
which is confirmed by the warning issued:
The “inhomogeneity field estimation” mentioned here is the step required for the correction of susceptibility induced distortions, which are typically most prominent in the frontal lobes, hence your observation:
When there is no reverse phase encoding direction information, dwifslpreproc can only perform motion correction, eddy-current distortion correction, and outlier detection and replacement (which is already quite a lot!). But it can’t deal with susceptibility-induced distortion without some way of estimating the distortions, which is normally via a reversed phase encoding b=0 scan.
I’m not sure there’s a lot you can do about this, though there are reports of successful distortion correction using other approaches such as registration to a T1 template – but my (limited) experience with these approaches has not been particularly compelling. The synB0-DISCO approach is also worth looking into if you really need to correct for these artefacts (not as strong a requirement for TBSS as it would be for tractography, to be fair).