For others who hit this issue, it has to do with servers not sending out caching headers. When hovering a button, an image which should be ready to display needs to download again. To fix this, add the following
.htaccess file to your document’s directory: https://gist.github.com/themorgantown/4758846.
To test what caching headers are being sent from your server, open Terminal and run this:
curl -I http://site.com/image.jpg
If your file is being cached, you’ll see something like this in the output:
Expires: Sat, 25 Oct 2015 23:07:42 GMT
More info on setting this up.