Recent posts on homepage: different thumbnail based on logged in status and post category

This works well on archive.php, but not working on the homepage. For some reason, the first statement doesn’t get applied: if a post is in Members category and user is not logged in, it’s supposed to show a default placeholder image, but instead it’s still showing the actual post thumbnail.

get_the_post_thumbnail_url does not return anything if image size is set

I want to display the post-thumbnail image in the header using get_the_post_thumbnail_url() with defined size, however if I am using it without specifying the size, like get_the_post_thumbnail_url() it is displaying the thumbnail img, but if I am using it like: get_the_post_thumbnail_url('thumbnail') nothing is shown.