I just cloned an email that I use every month; I just change the image and copy slightly. Today I cloned it to use again, but the template is practically blank. There is one element (the rep signature) at the bottom, but everything else is gone. I sent a sample of it to myself, and it looked fine, but it's impossible to edit when it's invisible! Anyone else run into this?
Since you are getting the apt content upon sending the sample It is very unlikely that your email code has been compromised it could just have been the site rendering issue or may be due to partial loading of the edit mail page. Try editing it in the incognito mode/different browser with cache cleared.