Form validation: blank fields bug?

[eluser]urrmurrmur[/eluser]
When I tried to extend the form validation library to add some validation of my own, I encountered some unexpected behaviour: my new validation rules only seem to get called on fields that are not left blank.

I initially thought my code was doing something stupid (I'm quite new to CodeIgniter) and made a topic in the code and application forums, but no one there could find a problem in my code. So, I was wondering if this might be a bug in the form validation library?

The full topic with my sample code in it can be found here.