Ok, here it is… I’ve tested the thing and failed to repeat the bug. That means there must be a conflict with some other plugin.
If I am to help you further to resolve this, you should send me the list of plugins you have activated that make front end changes in the article editor page, so I can try to determine the conflict source.
Ok, I am guessing that there is a server problem (settings, permissions) in the hosting location, which should explain the fact that all your instances show the same behaviour, one I cannot reproduce in none of my test or production servers. But, I really don’t have a clue what could be the problem.
We will schedule a Skype call then and troubleshoot live.