What is the meta key name of the field as you are using it in the form? If it starts with an underscore, that is the wrong key. Look for the key name without the preceding underscore.

Also note that any posts that does not have a value in this meta key will be excluded from the results.

Everything in the form setup seems OK, and the data on the posts is also OK. As the form is using the ‘As an Archive’ method, it may be that your theme template (category.php) is calling some code from within the theme that is overriding our sort.

You could test this by making a test page with a test form (a duplicate of the real one, but set to Shortcode method), following this guide:

https://searchandfilter.com/documentation/search-results/using-a-shortcode/

It will not look pretty, but might give some clues as to what is going on.