Authors delete posts, accounts get suspended or locked, and a public account can switch to protected. Any of those turns a bookmark into an empty row. The URL still looks like an address. It just does not resolve to the words you remembered.
That is a property of the product, not a glitch. X bookmarks, likes and embedded timelines all point at the live post. If you need the post after that, you have to have stored the content somewhere X does not control.