"Healing done by Ward of Flame applies a stacking buff, increasing the healing AND absorbtion of Ward of Fire by 30%, reducing the cast time of Ward of Fire by 0.5s and the mana cost by 5%, per stack. Max 6 stacks."

At zero stacks of Ward of Flame, and at max (6) stacks of Ward of Flame, the absorbtion value of Legendary Ward of Fire remains unchanged; only the Healing portion is correctly getting the 30% per stack applied.

Observed: Zero stacks, Ward of Fire Absorb = 40077, Heal = 57,780 (all non crit)
Six Stacks , Ward of Fire Absorb = 40077, Heal = 160,497 (all non crit)

Expected: As per the tooltip, both the absorb and heal values should be getting the 30% bonus per stack.

Repro Steps: Cast Legendary Ward of Fire with no stacks of Ward of Flame, cast Legendary Ward of Fire with 1+ stacks from Ward of Flame, observe the difference.

As an aside note, you also cannot macro "Legendary Ward of Fire." Command is not recognized as with other legendary abilities you can macro this way to test any difference there. The macro simply does nothing.