

I was editing my comment as you were responding. Check the issue on GitHub I linked in the edit, and maybe thumbs it up for visibility. One of the commenters mentions using a third-party tool but I’m not sure the one they linked to can grab posts. In theory another one might exist to dump your post data.
From what I read online, Matrix is not very good with its federated moderation tools. Apparently, a quirk of it can cause the state of a federated channel to reset in time, thus also undoing removing of messages. Here is a blog post criticizing Matrix.