For those having problems to catch up their booking on the MU website, try to add " MR" (or " MRS" ofc) at the end of your First Name. It solved my problem (same issues as described above by others) and allowed me to reserve my seats, with a ticket booked thru an OTA.
If it is not working, you can try to use the "Ticket Verification" section of their website to retrieve info about your booking. It worked better for me and was a way for me to notice : 1/ the " MR" issue with the first name, 2/ the MU PNR was different that the "airline PNR" given by the OTA. Be careful that, on this page, the ticket number must be written with the dash ("781-xxx").