diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index 4955fcc264a89..e5f174138de35 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -1146,9 +1146,23 @@ protected function process_blocks_custom_css( $css, $selector ) {
 		// Split CSS nested rules.
 		$parts = explode( '&', $css );
 		foreach ( $parts as $part ) {
-			$processed_css .= ( ! str_contains( $part, '{' ) )
-				? trim( $selector ) . '{' . trim( $part ) . '}' // If the part doesn't contain braces, it applies to the root level.
-				: trim( $selector . $part ); // Prepend the selector, which effectively replaces the "&" character.
+			$is_root_css = ( ! str_contains( $part, '{' ) );
+			if ( $is_root_css ) {
+				// If the part doesn't contain braces, it applies to the root level.
+				$processed_css .= trim( $selector ) . '{' . trim( $part ) . '}';
+			} else {
+				// If the part contains braces, it's a nested CSS rule.
+				$part = explode( '{', str_replace( '}', '', $part ) );
+				if ( count( $part ) !== 2 ) {
+					continue;
+				}
+				$nested_selector = $part[0];
+				$css_value       = $part[1];
+				$part_selector   = str_starts_with( $nested_selector, ' ' )
+					? static::scope_selector( $selector, $nested_selector )
+					: static::append_to_selector( $selector, $nested_selector );
+				$processed_css  .= $part_selector . '{' . trim( $css_value ) . '}';
+			}
 		}
 		return $processed_css;
 	}