If I use acf_register_block_type on the acf/init hook, as per https://www.advancedcustomfields.com/resources/acf_register_block_type/, the block doesn’t render in the frontend. If I use the init hook, it works. This appears to be a new problem, as my code was working fine in December.
I’m using WordPress 5.4, ACF 5.8.9.
I find that render_template and adding a template file doesn’t render, yet using render_callback and adding callback in functions.php (or referencing via functions.php) does.
This appears to have been resolved in a subsequent update, as both render_callback and render_template work on the acf/init hook using WordPress 5.5 and ACF Pro 5.9.