Weird, did you buy a grey import maybe? (Amazon Italy for example). It maybe also possible that they installed a refurb board which was originally from Italy, might be worth taking it up with them?
If you dont want to faff, then just download this: https://www.sammobile.com/firmwares/galaxy-s8/SM-G955F/BTU/ and install using odin.
I bought it from unshackled.com which is formerly Phones4U - does that count? It was sold as a factory unlocked UK S8...
Called Samsung, they said it's an Italian S8 originally locked to Vodafone. I guess that entitles me to return or replace it with the Vendor surely under mis-selling of goods?