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If everything is brand new and built by someone else, then it really isn't your problem. Sending it out of the shop when it wasn't shifting properly was pretty poor, to put it kindly.
What you've described sounds very much like an alignment problem: a weak spring would only affect shifting in one direction.
I had this issue with a friends new Ultegra equipped Bianchi, trying to dial in the shifting never quite worked as it should. Tried new cables, outers etc etc. In desperation changed the derailleur and this fixed the problem. Examination revealed a weak return spring in the original (new ) derailleur.