It was definitely not miscommunication.

The agent and I had initially thought it would not be possible to add the MX flight due to the reason you stated below, so she went off to check.

But came back saying it would have been possible if Cuba had not been involved. She told me to google for information relating to Cuba travel restrictions.

I did and found this relating to travel from LHR to HAV on a RTW:
"There are two key airlines in the round the world flights market that can be used to get between Cuba and the U.K. without going via the USA : Virgin fly from London to Havana on a direct non-stop flight twice weekly and Iberia have a daily connection via Madrid. The rest of this article will assume that you are flying round the world in a Westerly direction, though the same flights / conditions apply if you are flying in an Easterly direction.

If you fly out to Havana on Virgin, your next on that ticket will be out of L.A., San Francisco or Vancouver across the Pacific. To get there, 99% of people will need a separate bolt on ticket from Havana to Cancun (cheapest) or to Mexico City and make their way to the U.S. from there.

If you fly out to Havana on Iberia, you have the additional option of flying out on Iberia’s fellow One World carrier, Lan, who operate a once weekly flight between Havana and Santiago in Chile. Otherwise, the next cheapest option will be to add a bolt on ticket to Cancun or Mexico and make your way from there (Lan Peru have an interesting flight from Mexico City to Lima in Peru for example). If you are heading to the USA having started your trip with a flight out to Havana on Iberia, the key thing to remember is that you won’t be able use any American Airlines flights as part of your main round the world ticket."


Just curious, that's all!