Thanks for the reply, though the problem persists. I actually tried your solution before but I ran into the same problem.
res=((M12 // FermionSpinSum) /. DiracTrace -> Tr) // Contract
I still get a product of traces, because there are still dummy indexes inside them:
tr(...GA[alpha]...GA[alphalin]..) tr (... GA[alpha]...GA[alphalin]...)
Everything is multiplying, so there are no "free indexes" M12 is a Lorentz scalar.
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