When a moderator converts a post into a comment, they are deleted with no owner information. This shows on the 10k tools as:
Deleted yesterday by
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My proposal is that when an answer is converted to a comment, the system should automatically make a comment "on the mod's behalf" to put the comment it was converted to. Something like:
Converted to Comment on post XXXX ~ drachenstern timestamp
So it's obvious which post it went to (you could even hyperlink it with markdown) and we know that the last action was "convert to comment" so it can be tracked easily.
A workaround for this (and consequently, the related issue of no traceability) is to delete the post yourself first, then use convert-to-comment while it is deleted.
This will reset the timestamp of your former deletion to match the event of the conversion. It doesn't address the issue that it won't display normally, but if you do this going forwards it will show up properly in the deleted list and other relevant histories.