@Curve_Marie: Thank you! I had another transaction yesterday with the same issue. It was MCC 8099 and the bank see this as 8999.
What is the rationale behind these changes, why are they needed? I think this creates unnecessary uncertainty for your users.
Banks can invent various cash back schemes and the easiest way for them is to list the MCC codes for higher cash backs. That’s the reason you could have seen the need for MCC codes visible for transactions as a new feature request (MCC code and/or description shown in transaction details).
As an example Erste Bank Hungary Max card gives 4% for various categories that you can find on the last page in the below linked document. They have collected similar MCC codes into categories and you can pick 3 categories active at the same time.
In my case MCC 5941 would have meant 4% (second group), but as MCC was changed to 5999, it suddenly became standard 1%.
As these changes mean real money for your users, it would be great if you could disclose the mechanism, conditions, etc. If this is not possible, I think the community will sooner or later reverse-engineer the mapping logic.
Thanks again for your investigation, let us know if we can help you finding a good solution for this.