when you give yourself an Iron Sword like this
/give @p minecraft:iron_sword 1 0 {AttributeModifiers:[{AttributeName:"generic.knockbackResistance",Name:"generic.knockbackResistance",Amount:1,Operation:1,UUIDLeast:894654,UUIDMost:2872},{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:-0.35,Operation:1,UUIDLeast:894654,UUIDMost:2872}]}the Knockback Resistance seems to no longer work properly.
"no longer work properly" His behavior doesn't seems changed, you most likely don't know how it is working :
You command give a sword that double your knockback resistance, but the base knockback resistance for players is 0, so in this case, you won't block any knockback.