The only way I can duplicate this so far is by editing the original topic. There is definitely a problem where it doesn't have the calendar entry filled in when editing. So, if the original post is edited and the user doesn't reset the data it does disappear. The fix will be a bit complex for this but I'll get it working for 7.0.1.
Other than that, so far I can't duplicate it just by replying. I've looked in the code and on a reply the calendar table isn't touched at all.
