diff --git a/src/wp-includes/blocks/post-excerpt.php b/src/wp-includes/blocks/post-excerpt.php
index 4ed4edab95078..922ca5111f7a0 100644
--- a/src/wp-includes/blocks/post-excerpt.php
+++ b/src/wp-includes/blocks/post-excerpt.php
@@ -83,7 +83,7 @@ function register_block_core_post_excerpt() {
  * the excerpt length block setting has no effect.
  * Returns 100 because 100 is the max length in the setting.
  */
-if ( is_admin() ||
+if ( is_admin() && ! wp_doing_ajax() ||
 	defined( 'REST_REQUEST' ) && REST_REQUEST ) {
 	add_filter(
 		'excerpt_length',