diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index 7175d7a88747d..b2152fb6b8dd0 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -816,7 +816,8 @@ private static function get_viewport_breakpoint_value_in_pixels( $value ) {
 	 * the default breakpoints when no valid custom breakpoint is provided. When
 	 * only one breakpoint is valid, it remains keyed by its configured state and
 	 * uses a single max-width media query. When `tablet` is not larger than
-	 * `mobile`, it is removed.
+	 * `mobile`, or does not share an absolute or relative unit type with it, it
+	 * is removed.
 	 *
 	 * @since 7.1.0
 	 *
@@ -852,7 +853,17 @@ private static function sanitize_viewport_settings( $viewport_settings ) {
 		$sanitized = array( 'mobile' => $breakpoints['mobile']['value'] );
 
 		if ( isset( $breakpoints['tablet'] ) && $breakpoints['mobile']['px'] < $breakpoints['tablet']['px'] ) {
-			$sanitized['tablet'] = $breakpoints['tablet']['value'];
+			/*
+			 * A media query resolves `em` and `rem` against the browser's default
+			 * font size, so the order of a pair that mixes a relative unit with
+			 * `px` only holds at the 16px base assumed above.
+			 */
+			$mobile_is_absolute = str_ends_with( $breakpoints['mobile']['value'], 'px' );
+			$tablet_is_absolute = str_ends_with( $breakpoints['tablet']['value'], 'px' );
+
+			if ( $mobile_is_absolute === $tablet_is_absolute ) {
+				$sanitized['tablet'] = $breakpoints['tablet']['value'];
+			}
 		}
 
 		return $sanitized;
diff --git a/tests/phpunit/tests/theme/wpThemeJson.php b/tests/phpunit/tests/theme/wpThemeJson.php
index c2cda7bb158d0..5ef2d4d76e1ac 100644
--- a/tests/phpunit/tests/theme/wpThemeJson.php
+++ b/tests/phpunit/tests/theme/wpThemeJson.php
@@ -1248,6 +1248,49 @@ public function test_get_viewport_media_queries_omits_tablet_when_its_breakpoint
 		);
 	}
 
+	/**
+	 * @ticket 65865
+	 */
+	public function test_get_viewport_media_queries_omits_tablet_when_breakpoints_mix_absolute_and_relative_units() {
+		$this->assertSame(
+			array(
+				'@mobile'  => '@media (width <= 30em)',
+				'@desktop' => '@media (width > 30em)',
+			),
+			WP_Theme_JSON::get_viewport_media_queries(
+				array(
+					'mobile' => '30em',
+					'tablet' => '580px',
+				),
+				array(
+					'include_desktop' => true,
+				)
+			)
+		);
+	}
+
+	/**
+	 * @ticket 65865
+	 */
+	public function test_get_viewport_media_queries_keeps_tablet_when_breakpoints_mix_em_and_rem() {
+		$this->assertSame(
+			array(
+				'@mobile'  => '@media (width <= 30em)',
+				'@tablet'  => '@media (30em < width <= 48rem)',
+				'@desktop' => '@media (width > 48rem)',
+			),
+			WP_Theme_JSON::get_viewport_media_queries(
+				array(
+					'mobile' => '30em',
+					'tablet' => '48rem',
+				),
+				array(
+					'include_desktop' => true,
+				)
+			)
+		);
+	}
+
 	/**
 	 * @ticket 65596
 	 */