Future strikes says that any hit has a % chance to inflict a hit for 50 void damage 2 seconds in the future, but it doesn’t get proc’d by Smite or Smite’s Fissure which it should given the wording.
Other anomalous interractions are that Hammer Throw only procs future strikes once (even if it hits twice, once going out, once coming back), Abyssal Echoes doesn’t proc it either (but it is a dot, so therefore doesn’t hit).
Therefore Smite is the only spell that hits that doesn’t proc future strikes. Every other melee skill or spell that hits procs future strikes.
I think this has to do with another bug on smite. Smite hits don’t apply ailment effects like ignite/shock/poison etc. unless that ailment is from the smite specialization (ignites/shocks).
Future strike isn’t a hit either, it seems to register like a stacking DoT effect, which is why it’s probably not working here.