Why is Microsoft deleting questions about problems with Copilot?

Are you asking about your thread: https://learn.microsoft.com/en-us/answers/questions/6008085/co-pilot-invaded-onenote-and-destroyed-a-page-of-n?
That thread never got deleted. Only the AI answer got deleted because you rejected it.

FYI: https://learn.microsoft.com/en-us/answers/support/ai-first-overview#how-to-use-qa-first-answer

Let me as a volunteer moderator, not working for Microsoft, shed som light on this.

Posts to Microsoft Q&A can be deleted automatically by the system or manually by a moderator. The chief reason for automatic deletion is the considerable amount of spam which posted to the site (and many over sites of this kind). And by spam I mean that the posters are trying to pimp some product. Or trying to pimp a phone number to an alleged support site, where you will be a victim to a scam if you call. Some days there can be over hundred spam posts to Microsoft Q&A - of which the spam filter seems to trap the most of them. But the automatic deletion also hits a number of perfectly legit posts, and I and other moderators monitors deleted posts to resurrect these legit posts.

When it comes to manual deletion, we don't go and delete a post just because it criticises a Microsoft product. However, if the poster only offers criticism, but do not present any specific problem he or she wants help with, it is likely that we will delete the post as being off topic. Microsoft Q&A is not a discussion forum, but forum where people can ask to get help with Microsoft products.

In the particular case of your post, I decided that it was on topic for Microsoft Q&A, but not under the tag the AI had selected for you. Instead I changed the tag to one related to the site itself.