I found this easiest to replicate in chrome, so my directions for replication will be specific to it. In the admin section, I have the plugin enabled, I have a post with a recipe. I am going to the Edit Post page for this post. I bring up the Developer tools for Chrome (F12 on my default setup). I hit CTRL + F5 to force a reload of the page and show the correct content in the pane. Go to the Network Tab. From this tab, right click on the column headers and add the Column for Domain. Sort by domain. You can see a number of items that load forcing them to load from HTTP despite the admin section of the WordPress installation being browsed on HTTPS. If the SSL is in place on the same domain, you could use protocol-less urls, but this was just a thought of mine in passing.
The page I am on: