Single Template for Custom Post Type stops working after WordPress 5.4 update

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Nothing changed in WP 5.4 regarding singular post types. The template hierarchy remains the same.

E.g for a post type named book WP will try to load the first file it finds in this list:

  • {selected-custom-template}.php
  • single-book-{slug}.php
  • single-book.php
  • single.php
  • singular.php
  • index.php

Your problems cause is elsewhere, if you’ve followed the standard approach to templates you should see no difference.