I see some users indicating that they've received downvotes in the comments, and asking their downvoter to justify why they downvoted the question. Is this acceptable for discussion? Should this occur in the comments, or somewhere else?
The downvoter ought to feel absolutely no duty to reply to such a comment, although of course, the author of the question or answer is perfectly entitled to solicit a comment; politely.
In which case a single comment could be made by the downvoter, aimed at improving the question or answer. Perhaps further comments can be made following any edits.
Any further discussion ought to take place in chat, if at all.