As reported on the WP.org forum:

The selector .disabled-blog.options-reading-php .form-table tr:nth-child(2) to disable the blog settings on the reading settings page is not accurate enough. If one adds another section via plugin to this page with two options, the second option is always hidden.

Since another section has another .form-table, I suggest changing the selector to .disabled-blog.options-reading-php .form-table:first-of-type tr:nth-child(2)