In my experience advance seat selection on codeshares across different alliances is usually tricky, or not possible.

I would call Latam as that is who your contract is with. They may have a quota or some other way to resolve QR not seeing the seat eligibility of your ticket. But their trade website says:

"On codeshare flights marketed by LATAM, seats must be booked every time a customer requests so. Only in the case of system limitations for pre-booking a seat in any cabin (e.g. a seat is booked but not confirmed), the agent must instruct the customer to book the seat directly with the operating carrier at the time of check in at the airport. Important: Seats and other ancillaries are not available for sale on codeshare flights"

That is not entirely clear to me but you may be a victim of the aforementioned "system limitations" or non availability of ancillaries.

Oneworld status may also be a red herring here as you are not ticketed by a oneworld airline. QR's website says "oneworld benefits are available only to passengers on scheduled flights that are both marketed and operated by a oneworld member airline (marketed means there that must be a oneworld member airline's flight number on your ticket)".

If Latam can't resolve this you are probably left with accepting QRs unexpected offer of a discount or waiting until check in.