I experienced this once before, and thought maybe I was dreaming
The comments that do this also seem to have some weird property that causes them to be listed at the top of my list of recent comments list -- the import process records their time of import, rather than their time of creation, as the time on the comment. And the comment is listed in its "raw" form from the import file -- the IP address, time etc., are all listed in the body of the comment, instead of in separate fields. I mention this only because I figure it might be a clue to what sets these comments apart from other comments and allows them to somehow erase the posts when deleted.
Has anyone heard of this before? Any clue how to fix it??
I am running MT 2.661... but this problem previously happened before I installed the new version, when I was on version 2.5something. So it's not a version-specific issue, seemingly.