[2020] Void Blades buffing 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.