I came to report that Lunge is not working against Lagon, but that seemed to have been reported and acknowledged already. I may have a possible reason for this, but at least I faced another problematic behaviour while fighting him.
I was using LMB (move and attack) for my main attack (Rive), and I had quite a bit of difficulty navigating the area. The problem is that Lagon’s targeting box is way larger than the graphics indicate him to be. When trying to avoid his attacks by moving aside, you have a very small target area for your mouse cursor to be in when clicking to actually get your character to move, instead of just starting to attack. It seemed like at least half of the platform where you are fighting is overlapped by Lagon’s targeting box. Since I’m playing HC char, I didn’t test it too much, but I’m sure you get the idea.
How this may affect the Lunge skill? It may be the case that since you are already inside the targeting box, you can’t use lunge?