Funnily enough, I just had this exact discussion on Line 
I am pretty confident that all loot from Chests, Titans etc… (wherever RNG Loot Rolls are determined) is determined at the time of collection. (hence the small delay between tapping the chest & it starting to display the loot).
In this instance, you should hold off on opening it until after the updated Emblem Drop Rates goes live (2.5 hrs time).
It’s stated in the announcement:
So what that means is previously from Elemental Chests the Emblem Options were 10, 20, 30 or 50 emblems.
Post Update going live they will be: 20, 40, 60 and 100 emblems.
Source: PROJECT --- Elemental chests loot analysis