column values not showing after merging two custom post types

The correct action hook for printing custom column content is manage_loads_posts_custom_column, not manage_invoices_posts_custom_column (tested). This is because the row is itself for a loads post type, and not an invoices post type. Because you’re on the page for invoices, the filter to add columns works with invoices.

Tested:

add_action( 'manage_loads_posts_custom_column', 'invoice_custom_column', 10, 2 );

I received an error when setting the post_type parameter to an array, so I set to loads as a string. Additionally, pre_get_posts is an action, not a filter, so the return $query is not needed.