Thanks. It seems those are similar to the cheap UA fares from YYZ to south east Asia earlier this month - NH (or UA) metal but sold by UA. Those fares sometimes were bookable on UA.com if you searched using multi-city by segments (not this time). Less common but those tickets also possible sold by NH - usually slightly more than UA tickets (I think UA/NH have a JV transpacific.) However, those NH tickets were generally not bookable on NH website directly.
I recall fares like this a few times in the last few years. it is generally safe for a UA issued 016 ticket - never heard anyone of them got cancelled.
However, it is pretty much impossible to change those tickets through the OTAs - you'd better hope your plan is firm.