Thanks, @rootid – this turned out to totally be my problem too. I was echoing the content straight from the $post object like $post->post_content. What fixed it was switching to echo apply_filters( 'the_content', $post->post_content );.
The slightly misleading thing I found was that other blocks were appearing in post_content, just not my ACF custom blocks – I guess because of the order in which they are added – this hindered my diagnosis of the issue though, which I had assumed was a problem with my render_template path as the blocks were appearing correctly in the block editor.
