I have had a few instances where I have come across questions that I thought were particularly bad. These mostly involve a complete lack of research, and most of the time, I can answer the question with a copy-paste from official documentation (an answer which is then accepted). In some cases, these documents are linked with the original question, but the user reveals that they did not actually read them.
I try to leave a comment explaining my down-vote. In some cases, another user will reply to my comment to say that they do not agree with me, and will directly say they are upvoting to reverse my downvote. Other times, they will comment to say 'have an upvote to balance your score'.
In some cases, they have been actively commenting on the question, and thus far have not found any reason to upvote. With the additional comment that they are 'upvoting to reverse another user's downvote', is this behavior considered acceptable, or should such comments be flagged?
I do not want to single out specific users or sites, as it is irrelevant; I am wondering what the general policy would be for this sort of thing, in relation to general Stack Exchange policy.
Note that I am not asking if the actual practice is acceptable. I already know the answer to that. My question specifically asks about comments, which the suggested duplicate does not touch on, at all.