Custom Post Templates stopped working

hi there

I update recently my WordPress Version and now it seems that my custom post templates don’t get applied anymore…

I used the plugin (also quite old)
https://wordpress.org/plugins/custom-post-template/

What it does is that it allows to use a header like this

<?php
/*
Template Name Posts: Widescreen Post
*/
?>

and then there is a little dropdown next to the post one is editing where one should be able to select the post template.
That did work so far, but unfortunately even with a Template set (I think there is an entry in postmeta made in my DB saying which template it should use) the post falls back to the default template not doing anything of the code I define in the custom template.

Does anyone one what the reason for that could be. Maybe a solution?
Or another newer plugin?

Or is there maybe a built in wordpress function which allows the editor to select a template (post) per se?

Thanks very much for your help in advance.

Cheers