@barsmith
I just tried entering \( k^2 \) and it worked fine inside Chrome on Windows. Perhaps there was a network issue earlier in the day when you tried this and it couldn't load MathJax? It may have also been that there was a problem earlier in the day and Canvas fixed it.
To double check, you are trying this in a browser? The last I checked, direct entry of LaTeX doesn't work on the mobile app unless you have at least one equation on the page that is entered the normal way.
Another thing to check is to look at the HTML source for the page and make sure there aren't any stray characters that got introduced.
If you're still having the problem, can you try it on a new page? There's a possibility that there is a stray \( or \) somewhere else on the page that is messing the parsing up.