“Huh?” not just an interjection, it can be used as a question signaling communication difficulty. The problems in the case I described above are: first, that it doesn’t provide enough information about the type of communication difficulty, and second that once it’s repeated without providing further information it suggests the speaker is acting in bad faith.

If you’re speaking to me in a noisy room I might say “Huh?” to indicate that I can’t hear you. If we use different dialects or languages I might say “Huh?” to indicate that though I can hear you, I don’t understand your words. If we’re planning or actively coordinating something I might say “Huh?” to indicate that though I can hear you and comprehend your words, I don’t understand how your utterance relates to the task under discussion.

Personally I think there ARE such things as bad questions, such as those used for sealioning. If an underspecified question like “Huh?” is asked once or twice it might just indicate that the speaker is really confused, but there’s a minimal level of cooperativeness that’s needed for effective communication.

You’re mixing 2 layers. A question is a sentence, not a word. A sentence can consist of one or more words. For example “who” is a pronoun which can be part of a question or the whole question or part of a non question sentence. Even “me” can me used as a question: “You did that” – “Me?” This is a so called “echo question”. It doesn’t add new information, just echoes what’s already said.

So pointing out the part of speech doesn’t say anything about if it’s a question or not. Interjection is a part of speech. In this case, the interjection “huh?” is used as a question (notice the question mark). And to add another layer: It’s a discourse marker.

Even words that are grammatically a question cease to be a question if they aren’t really being used as one