Personal experience here.

There's a documented problem b/n UA and SQ for *A RTW tickets issued on SQ stock. It has been a pain in the butt for me and there's nothing I can do about it. Each time I had to find a SQ office in the US have my ticket reissued. And fortunately I haven't got stranded yet.

UA and SQ are talking but no end in sight. I am not sure of the exact problem but get the impression from my TA and ticketing staffs (UA and SQ) that it's UA who is being inflexible over the issue.

The fact that a *A RTW is A$200-300 cheaper when issued by SQ may have something to do with it.