Merchant codes (MCC) are selected by the merchant's acquirer, the company that processes their card transactions. It's unlikely that any customer facing staff at the restaurant will have any knowledge of this topic. Every card issuer experiences this problem if they offer category bonuses, because some merchants are misclassified, and it's not something the card issuer can control. Citi explicitly tell us this in the card terms:
Merchant Classification for Rewards Categories. Merchants are assigned a merchant category code (“MCC”), which is determined in accordance with Visa/Mastercard/American Express procedures based on the kinds of products and services the merchants primarily sell. We don’t control the assignment of these codes and are not responsible for the codes used by merchants. When you use your card to make a purchase, we're provided an MCC for that purchase. We group similar merchant codes into categories for purposes of making rewards offers. Sometimes you may expect a purchase to fit within a rewards category, but if the code assigned to the merchant wasn’t grouped into that category, as recognized by Citi, your purchase will not qualify for additional points. For example, you won't earn additional points for purchases at a restaurant located within a retailer if the restaurant is assigned a “retailer” code instead of a “restaurant” code. Please also note - purchases made through mobile/wireless technology may not earn additional points depending on how the technology is set up to process the purchase. We reserve the exclusive right to determine which purchases qualify for additional points.