The strange thing is, if you have the direct link, they show up. Examples:

https://mspabooru.com/index.php?page=post&s=view&id=145971

https://mspabooru.com/index.php?page=post&s=view&id=141161

Is that some kind of bug? I've already checked the blacklist, but there are no entries. How do I fix this issue?

This seems to be happening to several posts. For example, the tag "blazing_saddles" should have 10 posts, but only 5 will show up if you search for it.