Prism of Belief Bug

I found a bug with the Prism of Belief Diamond Jewel. It's supposed to increase the level of my Skeleton Sniper skill, but it's not working. My skill stays at level 31. I've tried removing the gem and putting it back in the socket, and I've also changed maps, but the jewel still isn't working. Can you help me figure out what's wrong?
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https://youtu.be/p5vNQMYLNPE