yes, I know, there is something weird.
-- Regarding efficiency: Please use ScalarProduct[p,p]=mt2 and ScalarProduct[k,p] = ... and ScalarProduct[k1,k1]=0 etc. (This will set also Pair[Momentum[...], Momentum[...]] and this is what is used inside OneLoop ). --
Try to do things without OneLoop. Just OneLoopSimplify. Or, probably better, use FormCalc, which will definitely be much faster than FeynCalc:
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