PHP files included in functions.php don’t seem to work

I always break my functions.php up into more managable, specific functionality files, for instance, all footer related functions goes into a file called footer-functions.php and pagination functions goes into a file called pagination-functions.php. This way, my code stays organised, managable and I don’t kill my functions.php. This however, is only related to theme specific functions, site specific functions goes into plugins in much more the same type of method

To come back to the point, at the end I include all my smaller specific function files into functions.php with require_once() as follow

require_once( 'footer-functions.php' );
require_once( 'pagination-functions.php' );

This is all you need. You should not hook this to any hook or add it in any function, it can have expected behavior which can lead to a broken site