The convention is that the first carrier usually does the ticket issue. If you specifically avoid the first carrier for issuing the ticket, then the one you choose may (and rightly so) only do the ticket issue if they get a fair bit of business out of your RTW - such as a Trans Pacific sector (hence the AA substituting for QF).

I am somewhat guilty with a soon to commence *A RTW in that TG issued the ticket but the only business they get is BKK-HKG (as first flight). In spite of that, they happily and efficiently have done the hack work.