Could you post the relevant part of the fare rules which details your claim, please?

My BA RTW rules for tickets issued in Sterling state that

"when the class of service for the fare is not offered book in the next lower class of service using the booking codes

A/D/M on all OneWorld airlines, except use
Y/Y/M on AY, EI LA & LP Domestic, and
A/C/Q on GF, and
J/D/V on PH

How do you interpret this to mean that your ticket should show Y class on a route where only Economy is offered?

I do 6 or 7 RTWs each year in A Class and have ALWAYS been put in D Class when there is no First Class offered (eg MIA-GCM, DFW-YYZ) and in M Class when only Economy is offered (eg SAN-LAX). Annoying as this is, particularly when M Class now accrues so little mileage/tier pts, it's been like this for years.

Please tell us where your ticket was issued and your starting point. Maybe there are different rules for different countries.