I just checked what the function you called does and it just retrieves the _wp_page_template value from the wp_postmeta table of WordPress. So probably the structure of your theme is not right, some rewrites are active, or something else (e.g. plug-in) altered this database table.
You could check that the theme root directory is indeed the directory which you think it is.
Other things that could be happening are:
The filename of a Page’s assigned custom template is stored as the
value of a Custom Field with a key named ‘_wp_page_template’ (in the
wp_postmeta database table). If the template is stored in a Theme’s
subdirectory (or a Parent Theme’s subdirectory of a Child Theme), the
value of the wp_postmeta is both the folder and file names
Which I got from here: https://codex.wordpress.org/Function_Reference/get_page_template_slug