Actual comments look like normal comment without upvotes.
What is this? An echo of migration from hashcode.ru?
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Upon investigating this issue further it does seem that these are leftovers from the migration.
The most recent voting history we have in our databases (that I could find) is for April 2015 (Technically, March 31st, but oh well). This means that we do not have the actual votes for these comments and thus I can't say where they came from with certainty.
The migration theory is supported by the fact that the most recent comment with a negative score is from March 12th 2015, if it was from answers being turned into comments there would be more recent ones.