Well, I wouldn't call it a bug...
FeynCalcExternal translates Eps[LorentzIndex[l1],...,Momentum[p1],...] into LC[l1,...][p1,...].
For this to make sense, the arguments to Eps must be a list of Lorentz indices followed by alist of momenta, not vice versa and not mixed. This can be achieved e.g. with Calc.
All in all, a matter of notation.
Best wishes, Frederik Orellana
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