Traditional Trundle Skin

I'd just recently played a game with a Trundle who had the Traditional skin. The shadows on his W, Frozen Domain kept glitching. What I mean is that when he cast it the shadows or dark tones on the skill would rapidly dim and brighten. I have no idea if it's supposed to be like this but when I had a team fight with him, the skills shadows did the rapid dimming and brightening which really confused me as we were fighting. I think it's a bug mainly because of how choppy and rapid the flickering was. It was pretty distracting too.