At some point, Riot secretly changed how Luden's echo procs on Large scale AoE
This Reddit thread states that the OP created a custom game as Karthus (quite possibly the only Large-scale AoE that is guaranteed to hit everyone) in which the Karthus procced on multiple champions not at the top of the scoreboard, but being the closest to Karthus. (There's evidence photos over in the reddit thread).
The weird thing is, according to the wiki, Large scale AoE is supposed to proc on whoever was First pick in champ select, but supposedly, at some point between Taliyah's release and Kled's release, it was changed to be based on Proximity, but was never mentioned by riot.
I'm just saying it's kinda unfair that Karthus can't control the luden's proc with his ult anymore. If Riot can give some context on this it'd be highly appreciated!