This problem can occur in 2 two situations:
1. If you have active additional language settings in the system.
To check it out please go to "Settings" => "Store Languages" and check the Translated Languages section.
If you have another language setup and don't use it in your store please delete this setup and try to send a test post. Link in the new posts will work.
2. If the product that was published in the post is no longer available.
Our system works in such a way that posts with an unavailable product are not published.
However, if at the time of publication the product that was chosen is available, but currently unavailable, users will go to the 404 pages when they click on the link.