I have this concept, although i never own an arowana, but i believe that the cause of droop eyes, is due to the diet & space.
1) Wild arowana do not consume food everyday, not everyday they get to find food, thus they are not kinda of overfed every single day.
2) Wild arowanas have plenty of space for them to swim / (exercise) thus the fats in the body should be lesser. which i find droop eye is kinda of fat deposits.
I believe that because of the limited space that we restrict domestic arowanas and the insane amt of food we give ( we always pamper our fishes do we?
) it may cause the droop eye prob.