Ok, I was still unable to reproduce on my test site.
I looked at the plugin list and notice you have a Post Order Plugin.
If you disable the Post Types Order plugin does it resolve this issue?
Post Order Plugins tend to play havoc with events as we order by date and those plugins tend to be aggressive in reordering everything to their custom order.
You maybe to use a plugin such as plugin logic to turn it off on the month view:
Try using the Plugin Logic to disable other plugins on the event views and single templates:
Plugin Logic
Let me know what you find out.
Thanks