The problem is in the representation. I am sorry to have understood it only now.
So from ?SUNDelta:
\\\\\\\"SUNDelta[a, b] is the Kronecker-delta for SU(N) with color indices a and b.\\\\\\\"
I think it is fair to assume that it means that the delta is in the fundamental, so the trace should be N, not N^2-1.
While actually the delta is in the adjoint. It would be nice to have it explicitly stated.
[by the way, if you don\\\\\\\'t know this, even simple computations like qqbar-->qqbar with a photon exchange give wrong results. And in general when you have QCD+EW things it is a mess.]
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