Thank you for the tip as I encountered similar issue.

Additional data points for anyone encountering a similar issue. I booked the multi-city itinerary under Basic Economy fare family and also paid for advanced seat selection. I called in hoping to cancel the itinerary and rebook a new one but the additional payment paid for seat selection meant that I would not be eligible for the Flexible Cancellation (cancel within 24 hours of ticketing). Unsure if this is actually true of if the agent was trying to make his life easier. Fortunately, I was able to just pay the difference in fare between Economy Classic and Basic Economy but essentially ate the cost of the paid seat selection.

When managing booking after the tickets were reissued to select the seats, as the Economy Classic fare comes with free seat selection, I am still prompted to pay for advanced seat selection. I will likely call back in and see if I can select seats through the phone.