devtrackers.gg
This revealing actually only got fixed for Lux E a little while (year or so) ago.
"Targeting" enemy units with single-target, area, pointblankAOE, or cone targeting type while the enemy team has no vision of you creates a reveal bubble on you at the start of the cast (when the engine-level targeting happens), unless the spell has this turned off with a parameter.
The actual "targeting area" for area and cone spells is typically NOT what's actually used by the spell script afterwards; It's just a leftover of a type of targeting that they never fixed to even work with cast times (so before they were each changed to just do this at script level, cone spells like Darius E would "find" their targets on cast start, and then affect them at the end of the cast time - wherever they went).
Anyway, since this whole emerging reveals stuff is super obscure and controlled by spell data that isn't directly maintained or even controls anything besides the revealing* (*new AI Bots might rely on this again too), that's why you get all these inconsistencies between spells and champions.
Ifyou ever wondered why Xerath W reveals you if an enemy was in the center circle specifically at the start of the cast, this is why. And since practically nobody, not even devs really know about this, it's always met with a mix of confusion and "maybe/definitely it's intended" in comment sections on reddit and youtube.