I had seen this question on HNQ, and after having read the question and existing answers, I left a comment on the question asking about what whether the parents had spoken to a doctor about the situation. At the time, there was already an extended conversation in the comments that was unrelated to what I was asking. However, to my surprise my comment was moved to chat along with the rest of the discussion. Initially I thought this was a mistake, so I left another comment asking the moderator why this decision was made, but that comment was deleted, so it seems this was not accidental. Why was my comment moved to chat as well? Did it break some kind of guideline that I was not aware of? If so, why wasn't it deleted directly?
Comments are for clarification, that's it, really, especially on an HNQ, which can get 40+ comments.
Your comment was partly asking for clarification and mostly an answer (by suggestion and by direct answer), so it was moved to chat with the others.
Every comment after a thread has been moved to chat is deleted, especially meta comments like "Why was my comment deleted?" This is the place to ask that question, not on the main site and when it has nothing whatsoever to do with the OP's question.
If you have other questions, this is the place to ask.
Just to add something to @anongoodnurse's excellent answer: Once a set of comments has been moved to chat, no more comments can be on that particular post (question or answer). So, once we've chosen to move the comments to chat, the expectation is that chat is where you go if you want any further clarification or really anything else - we can't move anything else to chat, and so the only thing we can do is delete.
Comments are a part of StackExchange that are necessary, but are not really well implemented (though probably as well as they could be). Comments don't display well, they aren't really easy to manage, and they are very hard to read. For that reason, we ask that they be used sparingly - and any comment is susceptible to being removed at any time, even if it has some value on its own, because that's often the only way we can manage the sheer number you get on a HNQ.
We will sometimes try to preserve clarifying comments, but it's not something you should expect normally. It's hard to do - our choices are either mark every single comment that we want to delete, or move all of them, and on a HNQ in particular that's usually tens of comments - a lot of work to mark individually. So unless there's a comment that's particularly valuable to preserve, we won't. I wish there was a better way to do it, but there really isn't (without using some sort of third party tool, anyway, which some mods do use but isn't default, and I haven't found one that handles this well.)