Yesterday I downvoted a question for being unclear. Today I received a notification "Question Undownvoted".
Does the system undownvote questions/answers automatically? Did someone vote it up and was my downvote removed?
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Why down votes are automatically removed?
Only when "the system" has detected suspicious voting behavior.
When, for example, you do targeted voting, the system might observe: "user X is quickly down(up)voting multiple posts of user Y", that might be determined to be serial voting, and the system automatically undos all corresponding votes.
In other words: the system "automatically" undoing your votes is most likely based on specific (unwanted) behavior on your side.