The travel agency uses Galileo to make the live booking. However, the booking is also stored in separate PNRs residing on each operating airline's own reservation system (in this case Amadeus).

When the MPC number is entered into the Galileo PNR and saved, it also gets transmitted to Amadeus (which MH uses).

The CX/MPC website does a query to Amadeus when you login and asks it to display all reservations that have your MPC number attached. Even though the Amadeus PNR has no CX sectors, it would still show up as a result.

It might be possible that some airlines using Amadeus may have setup certain security features in their PNRs which would prevent another carrier also using Amadeus from displaying it.