For the 3dTstat -argmax to work out right like this, you would need a "blank" zero dataset to be there first; otherwise, the first ROI is set to zero.
3dcalc -a roi_01.nii.gz -expr '0' -prefix roi_00.nii.gz
You could also use "3dTstat -argmax1" to set any ROI to at least 1, then mask the dataset with the sum across the ROIs.
3dTstat -sum -prefix anyroi.nii.gz AllROIs+tlrc.HEAD
3dcalc -a anyroi.nii.gz -b VisAllROI+tlrc -expr 'step(a)*b' -prefix VisAllROI_masked.nii.gz
There might be a typo somewhere here, but you should get the gist.