diff --git a/wp-includes/l10n.php b/wp-includes/l10n.php
index 1923c8c2a2..632f432f62 100644
--- a/wp-includes/l10n.php
+++ b/wp-includes/l10n.php
@@ -821,7 +821,7 @@ function load_textdomain( $domain, $mofile, $locale = null ) {
 	if ( 'mo' !== $preferred_format ) {
 		array_unshift(
 			$translation_files,
-			substr_replace( $mofile, ".l10n.$preferred_format", - strlen( $preferred_format ) )
+			substr_replace( $mofile, ".l10n.$preferred_format", - strlen( '.mo' ) )
 		);
 	}
 
diff --git a/wp-includes/l10n/class-wp-translation-controller.php b/wp-includes/l10n/class-wp-translation-controller.php
index fbe5fa7d0c..616dce5793 100644
--- a/wp-includes/l10n/class-wp-translation-controller.php
+++ b/wp-includes/l10n/class-wp-translation-controller.php
@@ -151,11 +151,13 @@ final class WP_Translation_Controller {
 		}
 
 		if ( null !== $locale ) {
-			foreach ( $this->loaded_translations[ $locale ][ $textdomain ] as $i => $moe ) {
-				if ( $file === $moe || $file === $moe->get_file() ) {
-					unset( $this->loaded_translations[ $locale ][ $textdomain ][ $i ] );
-					unset( $this->loaded_files[ $moe->get_file() ][ $locale ][ $textdomain ] );
-					return true;
+			if ( isset( $this->loaded_translations[ $locale ][ $textdomain ] ) ) {
+				foreach ( $this->loaded_translations[ $locale ][ $textdomain ] as $i => $moe ) {
+					if ( $file === $moe || $file === $moe->get_file() ) {
+						unset( $this->loaded_translations[ $locale ][ $textdomain ][ $i ] );
+						unset( $this->loaded_files[ $moe->get_file() ][ $locale ][ $textdomain ] );
+						return true;
+					}
 				}
 			}
 
@@ -163,6 +165,10 @@ final class WP_Translation_Controller {
 		}
 
 		foreach ( $this->loaded_translations as $l => $domains ) {
+			if ( ! isset( $domains[ $textdomain ] ) ) {
+				continue;
+			}
+
 			foreach ( $domains[ $textdomain ] as $i => $moe ) {
 				if ( $file === $moe || $file === $moe->get_file() ) {
 					unset( $this->loaded_translations[ $l ][ $textdomain ][ $i ] );
@@ -185,18 +191,21 @@ final class WP_Translation_Controller {
 	 * @return bool True on success, false otherwise.
 	 */
 	public function unload_textdomain( string $textdomain = 'default', string $locale = null ): bool {
+		$unloaded = false;
+
 		if ( null !== $locale ) {
-			foreach ( $this->loaded_translations[ $locale ][ $textdomain ] as $moe ) {
-				unset( $this->loaded_files[ $moe->get_file() ][ $locale ][ $textdomain ] );
+			if ( isset( $this->loaded_translations[ $locale ][ $textdomain ] ) ) {
+				$unloaded = true;
+				foreach ( $this->loaded_translations[ $locale ][ $textdomain ] as $moe ) {
+					unset( $this->loaded_files[ $moe->get_file() ][ $locale ][ $textdomain ] );
+				}
 			}
 
 			unset( $this->loaded_translations[ $locale ][ $textdomain ] );
 
-			return true;
+			return $unloaded;
 		}
 
-		$unloaded = false;
-
 		foreach ( $this->loaded_translations as $l => $domains ) {
 			if ( ! isset( $domains[ $textdomain ] ) ) {
 				continue;
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 10606651d3..18d8cb6360 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-57338';
+$wp_version = '6.5-alpha-57339';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.