Hi there – the reply I received from ACF is below. I had actually got it to work by amending the template file header, but assumed that the ACF plugin would be able to edit that itself automatically:
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Hi there,
Thanks for reaching out to us and sorry for the massive delay on this one.
In order to render the Template dropdown, you need to add the CPT key/slug to the custom template file header as described here: https://developer.wordpress.org/themes/template-files-section/page-template-files/#creating-page-templates-for-specific-post-types.
Template Post Type: custom post key/slug here
That way the Template dropdown will appear on the Attributes section.
Please try this and let me know how it goes.
Thanks,
Said
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Hope that helps.