You are observing the same thing that I saw in my data, and I believe the issue is the lack of any cluster-based threshold.  I believe the pFDR is voxel-wise only.  

To address this, I found Simon Eckhoff's SPM ANATOMY toolbox helpful.  With this toolbox, one can select the pFDR thresholded "neg" or "pos" beta difference maps and then apply a cluster extent.  Albeit this cluster extent is arbitrary and limited to voxel count alone, you can at least make a large cluster threshold to minimize Type II error (e.g., I went with voxel pFDR<0.001 and k > 200 in my data).  Note, if you use the Anatomy toolbox, you will need to apply a "-1" multiplicative transform to visualize the "neg" beta difference map blobs.

Hope this helps,
Jeff