In the miscellaneous quest "Find the word of power in Arcwind Point" I found and learned the word of power but it's still not cleared as completed. Any ideas what's the issue?

http://www.uesp.net/wiki/Skyrim:Arcwind_Point\

Read the bottom under Bug's... It's a known issue.

As mentioned, it's a well known bug.

If you're on PC or Xbox One, you can correct it with the Unofficial Patch.

Otherwise, just ignore it.