Justice is a fan-made global mod.

Why not living – because it’s not made of blood and flesh, as stated in the description of the “Living creature” parameter.

It has flesh, but instead of blood, it has sap...

And it’s strange that vampires suck birch sap and restore their health.

Well, it’s not birch sap. Since Ents have their own specific characteristics (a mobile plant, not an ordinary one), it makes sense to assume that their sap differs from the sap of ordinary trees enough for vampires to feed on it.
For example, photosynthesis doesn’t provide enough energy for movement => in addition to photosynthesis, they need to breathe actively => the sap is rich in oxygen and, in terms of properties, is somewhat more similar to blood.

And regarding the element of the Ent – water and earth together create wood.

As far as I know, in Ashan, all elementals are single-elemental.