What if it is one of the moderators or some moderators that is deleting it. Some posts are reported and got deleted. You can see some posts that re reported to be many and belonging to one user and moderators keep deleting it.

If someone is intentionally deleting his post, go to the person posting history and see if he is a bounty hunter. He may want to change and get merit to get a signature campaign. You can check if the person account password change or email changed, which means the account has been bought.