WP / ACF: Ensure ACF fields can be previewed when post revisions are disabled.

acf-preview-changes-when-revisions-disabled.php

<?php

/**
 * Ensure ACF fields can be previewed when post revisions are disabled
 * (`WP_POST_REVISIONS`).
 *
 * @listens action:wp_revisions_to_keep
 *     Filters the number of revisions to save for the given post.
 */
add_filter( 'wp_revisions_to_keep', function ( int $num, WP_Post $post ) : int {
	// Bail early if post revisions enabled.
	if ( WP_POST_REVISIONS ) {
		return $num;
	}

	// Bail early if the post type is not supported.
	if ( ! post_type_supports( $post->post_type, 'revisions' ) ) {
		return $num;
	}	

	// Bail early if not previewing post.
	if (
		! is_preview() ||
		empty( $_GET['preview_id'] )
	) {
		return $num;
	}

	$preview_id = (int) $_GET['preview_id'];

	// Bail early if not previewing this post.
	if ( $preview_id !== $post->ID ) {
		return $num;
	}

	// Bail early if not currently filtering the post ID
	// for ACF to fetch from its latest revision.
	if (
		! doing_filter( 'wp_revisions_to_keep' ) ||
		! doing_filter( 'acf/validate_post_id' )
	) {
		return $num;
	}

	return 1;
}, 10, 2 );