There’s a vague tooltip in Hengestone wherein your character will get its HP doubled, regardless of your transformation, in this case, both Lycanthropy and Transform will give your character x2 HP as well as attack boost. The only difference between the two is that:
Lycan gives +10% more final damage ( 30% overall because of the 20% from Hengestone’s effect ) and knockback/knockdown resistance, while Transformation has the Beast (Bonus Crit) /Insect (Bonus DEF/MDEF) /Plant(Bonus HP/SP Reco) & Small (Bonus Evasion)/Medium (Bonus Attack from S/M/L mobs)/Large (Bonus +75% HP at max level) attribute thingy.
As long as you are under the effect of Hengestone, you will get the double HP bonus each time you transform ( NOT STACKABLE ).
Let’s say I used Lycan while under the effect of Hengestone. By then, I’ll get +100% HP/+30% Final Damage/+Crit Rate. But then I happened to click on Transform’s Hotkey while still being under the effect of Hengestone+Lycan. With that, I’ll still get +100% HP/+20%Final Damage/+Crit Rate/+ the bonus from Druid Attributes depending on the mob you shapeshifted.
It is not a bug, as it is intended for your character to double its HP
And I quote “Summon the Henge Stone. Increases skill levels of magic circles and installations by cleric type clases by 1 and stats while in the transformed mode.” By stats, they were referring to the Bonus HP Buff, Crit Rate, and Final Damage