diff --git a/src/wp-includes/l10n.php b/src/wp-includes/l10n.php
index 1923c8c2a2..632f432f62 100644
--- a/src/wp-includes/l10n.php
+++ b/src/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/src/wp-includes/l10n/class-wp-translation-controller.php b/src/wp-includes/l10n/class-wp-translation-controller.php
index fbe5fa7d0c..616dce5793 100644
--- a/src/wp-includes/l10n/class-wp-translation-controller.php
+++ b/src/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/tests/phpunit/tests/l10n/wpTranslationController.php b/tests/phpunit/tests/l10n/wpTranslationController.php
index 9e33a1f543..5a7206f6d9 100644
--- a/tests/phpunit/tests/l10n/wpTranslationController.php
+++ b/tests/phpunit/tests/l10n/wpTranslationController.php
@@ -105,6 +105,30 @@ class WP_Translation_Controller_Tests extends WP_UnitTestCase {
 		$this->assertTrue( $unload_php_successful );
 	}
 
+	/**
+	 * @covers ::load_textdomain
+	 *
+	 * @return void
+	 */
+	public function test_load_textdomain_prefers_php_files_by_default() {
+		$load_successful = load_textdomain( 'wp-tests-domain', DIR_TESTDATA . '/pomo/simple.mo' );
+
+		$instance = WP_Translation_Controller::instance();
+
+		$is_loaded = $instance->is_textdomain_loaded( 'wp-tests-domain', 'en_US' );
+
+		$unload_mo  = $instance->unload_file( DIR_TESTDATA . '/pomo/simple.mo', 'wp-tests-domain' );
+		$unload_php = $instance->unload_file( DIR_TESTDATA . '/pomo/simple.l10n.php', 'wp-tests-domain' );
+
+		$unload_successful = unload_textdomain( 'wp-tests-domain' );
+
+		$this->assertTrue( $load_successful, 'Translation not successfully loaded' );
+		$this->assertTrue( $is_loaded );
+		$this->assertFalse( $unload_mo );
+		$this->assertTrue( $unload_php );
+		$this->assertTrue( $unload_successful );
+	}
+
 	/**
 	 * @covers ::load_textdomain
 	 *
@@ -296,6 +320,21 @@ class WP_Translation_Controller_Tests extends WP_UnitTestCase {
 		$this->assertFalse( $is_loaded_after );
 	}
 
+	/**
+	 * @covers ::unload_file
+	 * @covers ::unload_textdomain
+	 *
+	 * @return void
+	 */
+	public function test_unload_non_existent_files_and_textdomains() {
+		$controller = new WP_Translation_Controller();
+		$this->assertFalse( $controller->unload_textdomain( 'foobarbaz' ) );
+		$this->assertFalse( $controller->unload_textdomain( 'foobarbaz', 'es_ES' ) );
+		$this->assertFalse( $controller->unload_textdomain( 'default', 'es_ES' ) );
+		$this->assertFalse( $controller->unload_file( DIR_TESTDATA . '/l10n/fa_IR.mo' ) );
+		$this->assertFalse( $controller->unload_file( DIR_TESTDATA . '/l10n/fa_IR.mo', 'es_ES' ) );
+	}
+
 	/**
 	 * @covers ::load_textdomain
 	 * @covers ::unload_textdomain
diff --git a/tests/phpunit/tests/l10n/wpTranslationsConvert.php b/tests/phpunit/tests/l10n/wpTranslationsConvert.php
index 0de159b609..320d15da77 100644
--- a/tests/phpunit/tests/l10n/wpTranslationsConvert.php
+++ b/tests/phpunit/tests/l10n/wpTranslationsConvert.php
@@ -28,7 +28,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 	}
 
 	/**
-	 * @covers ::unload
+	 * @covers ::unload_textdomain
 	 *
 	 * @return void
 	 */
@@ -40,7 +40,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 
 	/**
 	 * @covers ::load
-	 * @covers ::unload
+	 * @covers ::unload_textdomain
 	 * @covers ::is_textdomain_loaded
 	 * @covers ::translate
 	 * @covers ::locate_translation
@@ -62,7 +62,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 	}
 
 	/**
-	 * @covers ::unload
+	 * @covers ::unload_file
 	 * @covers WP_Translation_File::get_file
 	 *
 	 * @return void
@@ -77,7 +77,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 	}
 
 	/**
-	 * @covers ::unload
+	 * @covers ::unload_textdomain
 	 * @covers ::is_textdomain_loaded
 	 *
 	 * @return void
@@ -235,7 +235,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 
 	/**
 	 * @covers ::load
-	 * @covers ::unload
+	 * @covers ::unload_file
 	 * @covers ::is_textdomain_loaded
 	 * @covers ::translate
 	 * @covers ::translate_plural
@@ -289,7 +289,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 	 * @covers ::set_locale
 	 * @covers ::get_locale
 	 * @covers ::load
-	 * @covers ::unload
+	 * @covers ::unload_file
 	 * @covers ::is_textdomain_loaded
 	 * @covers ::translate
 	 * @covers ::translate_plural
@@ -333,7 +333,7 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 	}
 
 	/**
-	 * @covers ::unload
+	 * @covers ::unload_textdomain
 	 *
 	 * @return void
 	 */
@@ -411,7 +411,6 @@ class WP_Translation_Controller_Convert_Tests extends WP_UnitTestCase {
 
 	/**
 	 * @covers ::load
-	 * @covers ::unload
 	 * @covers ::is_textdomain_loaded
 	 * @covers ::translate
 	 * @covers ::translate_plural