Custom page template is selected but the page uses the default

I’ve created a custom page template called “Archives Index” and created a blank page called “Archives Index” with the slug “archives-index” and selected my custom template in the Attributes box on the Edit Page page.

Problem is, when viewing the page the default page.php template file is used, rather than my archives-index.php file. Any ideas why this might be occurring? I can’t figure it out and I can’t find anyone having similar issues despite many searches…

Many thanks in advance