Paarthurnax Fire Breath Word won't come off wall

[MidbossVyers]

After finishing the main quest, I figured I would try to get the rest of the Words of Power, so I checked what I needed, and I noticed that I still needed one more Word for Fire Breath, but I had already completed both the main quest, the Companions quest line, and visited Sunderstone Gorge. I checked Sunderstone and Dustman's Cairn and found nothing. Then I went to Paarthurnax at the Throat of the World and noticed that the Word on his Word Wall was still glowing. I tried to unlock it by going near it to the point of touching it, but it didn't come off, and as a result, I didn't learn it. How do I get it? Is there a fix for this?