Problem:

I created 2 custom fields of type "Select" in this post type.
Options of these 2 select fields must be the titles (text) and postid (value) of all available projects.

BUT the select fields each show only 5 options, while there are 8 published projects.

Solution:

Please try to add Pagination limitation parameter in $args, for example:

https://toolset.com/forums/topic/dynamic-population-of-select-only-5-options-are-loaded/#post-1762209

Relevant Documentation:

https://developer.wordpress.org/reference/classes/wp_query/#pagination-parameters