That interaction is DoTs.
As it currently stands, Dark Harvest procs off ANY damage source that is applied to a target, including ignite, Luden's procs, Red Smite's true damage, and Liandry's. It stands to reason that it could proc off Scorch as well.
I'm worried because this could lead to a good number of unintended collateral kills in which the keystone itself is oppressive because the counterplay is boiled down to, "don't get hit with DoT." and it seems in specific cases it'd be easy to hit 2 targets -> proc DH on one -> kill the first -> proc DH on the other purely off of something like Brand passive.
This is also pretty cheesy on champions like Kha'Zix, who I saw hit a champion from ~65 -> 15% with a Q and then Dark Harvest finished them off on Red Smite DoT.
I think it's worth looking into and nerfing so that it only procs of the initial hit/tick (if you get ignited, it only procs with the first application of ignite) or lock the keystone to not proc off item effects (Luden's, Liandry's, Skirmisher's). This interaction only serves to make the keystone better for mages (and Kha'Zix) than the champions it was designed to be used by.
Then after that, it could stand to get some buffs and/or retooling on the maps that aren't ARAM. Perhaps reducing the %hp threshold and returning some of the pre-hotfix damage.