Was the contribution actually excess or was it a permissible contribution but you just decided it wasn't in your best interest? If it wasn't actually an excess elective deferral, I think that elective deferrals are irrevocable, so I'm not sure that you were permitted to just "undo" the contribution and it should instead be treated actually contributed and as a regular distribution despite the coding on the Form 1099-R. But I suspect that the IRS is unlikely to catch that and the IRS is likely to just go by the coding on the Form 1099-R.
As a return of excess deferral, I agree that there should have been two 2018 Forms 1099-R, one code P for $13,000 with $0 taxable and one code 8 for $357 with $357 taxable.
