I was doing some testing & it appears that Void Blades is buffing the damage for spell-proc’d Future Strikes but not melee-proc’d Future strikes. Void Bolts appears to be working as expected (increasing the damage spell-proc’d Future Strikes does but not melee-proc’d Future Strikes).
Without either of them, I was getting an average of 106.5 damage from melee-proc’d Future Strikes & 172.6 from spell-proc’d Future Strikes.
With 8 points in Void Blades (+24 melee void damage), I would expect increased damage on melee-proc’d Future Strikes & unchanged spell-proc’d Future Strikes. What I saw (granted with a small sample of ~20) was an average of 104.7 damage from melee-proc’d Future Strikes & an average of 180 from spell-proc’d Future Strikes.
Future Strikes don’t benefit from added damage, only from Increased Damage or Increased Void Damage. I did test the Void Blades and Void Bolts nodes on a fresh character and didn’t see the behavior you described.
If you have any conditional effects that grant Increased Damage or Increased Void Damage that could cause it, but the flat damage nodes are working as intended.
That’s the thing though, the only passives that changed were the presence or absence of Void Blades/Bolts.
Ah, though I do have an “increased damage if you have cast Decouring Orb recently” idol & DO was the one I was using to check the spell-proc’d Future Strikes damage…
