If the user enters a newline character in a title of a subpage, then the page would not be linked correctly to the mainpage.

Example:

[[ Hello my nice
World]]

This would create a wiki_pages entry with the title "Hello my niceWorld" with the result, that the entry in wiki_links would not be updated to be linked to the correct page.

Probably one solution could be to replace the newline character with a space in the mainpage and the subpage title.

Or another approach is, to remove these newline characters from the mainpage, if the are in a subpage link.