Gah, right, Jack Frost was an advent. Saint Nicholas was the one I was thinking of. The first raid after Jack o' Lantern. The bonus soul drop rate increase definitely happened with Saint Nicholas, as I distinctly remember not even bothering to get everything with Jack o' Lantern (my SSR weapon from that is only 1* and I needed luck to max out the eidolon), but I managed to max out Saint Nicholas's stuff with I think a similar, or not much higher, level of commitment.

And correct, that instance of Barong was one I started.