While doing a request to the comments URL using comments?tags=id:[post_id] the response is correct
However, while doing the same request with comments.json?tags=id:[post_id] the response is the post's information, not the post's comment information

e.g.
https://e621.net/comments?tags=id:2365945 -> returns comment information as HTML(expected)
https://e621.net/comments.json?tags=id:2365945 -> returns post's information as JSON(unexpected, comment info expected)

In another words:
posts.json?tags=id:[post_id] == comments.json?tags=id:[post_id]
only on *.json calls

i.e.
https://e621.net/posts.json?tags=id:2365945
https://e621.net/comments.json?tags=id:2365945
issue the same response

Not sure if this is a direct result of using non-wiki-defined behaviour(to the best of my knowledge)

rolling_atackk said:
https://e621.net/comments.json?tags=id:2365945 -> returns post's information as JSON(unexpected, comment info expected)

see danbooru's help:api

help:api said:
The base URL is GET /comments.json.

• group_by Must be: comment, else it will return posts.

try:
https://e621.net/comments.json?tags=id:2365945&group_by=comment

Thanks for putting me in the right track. Though your answer did not produce the solution I was expecting, it did allow me to stumble upon this new URL:

• https://e621.net/comments?format=json&group_by=comment&search[post_id]=2365945

Which correctly returns the comments as a JSON document.

I have another question, completly unrelated to the API calls. What does it mean for a post to be bumping?

Thanks again for helping me ;)