As far as I can see there's nothing wrong with DiracOrder.
What's happening is just that (see http://www.feyncalc.org/Polarization/index.html) the contraction of a momentum frou-vector and the corresponding polarization vector is automatically set to zero.
That is, transversality is assumed. If you don't want that, use some other name than Polarization[...] for the polarization vector.
Hope this helps.
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