custom templates exist but get_page_templates() does not function

Ahoj!

i built several custom templates for pages and could use them in my theme, so some subpages have the right custom pages templates. All of the sudden the option to use them is missing – only the default template can be found by get_page_templates(). i didn’t update or delete anything from my content themes folder.
>> edit-page-form.php:
function page_template_meta_box($post){ ***only the Default template is found***}

surprisingly my earlier built subpages still work fine with the custom templates.

my questions:
where and how does get_page_templates() check for the templates?
is there an ID linking the subpage to the custom page template? i checked the database without success. (only found parent page ID)

thank you for your help.
Luise