Ok, I'm not 100% certain that this is the plugin (I saw this happen on someone else's site), but it looks like this plugin doesn't roundtrip the HTML encoding properly.
If I type "test" into the original comment field and surround it with escaped angle bracket entities (which this forum thing won't let me post here), this is stored literally and then displayed as "<test>" in the actual page, as expected.
If I then go to edit it, it's now shown as "<test>" in the comment edit field. If I don't re-encode this before saving then it will get treated as an actual HTML tag (and become invisible).
Most likely, you need to call htmlspecialchars on the comment text before inserting it into the textarea control.