I'm not at a computer where I can access the files, but I just noticed that all the posts written before I converted have "Last edited by <blank> on <1969-... >" which is why even silent edits are just being updated with blank (or real values if the mark is removed from the Silent Edit option.
So the error isn't in the silent editing but in the fact that posts that weren't edited (but simply posted one time only) got the last edited by value set during the conversion. I'm thinking the fix would then be to update the database somehow. Does this make sense?