This will be the same reason as most other procs that don’t work on Lagon. LE uses the base of a mob’s model to determine whether or not it is in range. Since Lagon’s base is a long way beneath the player he’s out of range & therefore the proc you mention won’t work.
Reign of Dragons shouldn’t be an issue, you can try it with the Primeval Dragon as it uses the same model but does significantly less damage.