Wrong language used for inherited field value

The change in the MR fixes the problem for me. I am not sure if this is the right solution though. We are using a computed field, and these are not usually marked as translatable. However, reading the code in ContentEntityBase::getTranslatedField() it looks like if a field is not marked as translatable, then a copy of the field using the default language is always returned:

      // Non-translatable fields are always stored with
      // LanguageInterface::LANGCODE_DEFAULT as key.
      $default = $langcode == LanguageInterface::LANGCODE_DEFAULT;
      if (!$default && !$definition->isTranslatable()) { // <<<<<<<< If the field is not marked as translatable, the default language version is used for all translations.
        if (!isset($this->fields[$name][LanguageInterface::LANGCODE_DEFAULT])) {
          $this->fields[$name][LanguageInterface::LANGCODE_DEFAULT] = $this->getTranslatedField($name, LanguageInterface::LANGCODE_DEFAULT);
        }
        $this->fields[$name][$langcode] = &$this->fields[$name][LanguageInterface::LANGCODE_DEFAULT];
      }

Unfortunately I couldn't write a test for the module in the time I had available, but here is a rough draft of a test that showcases the problem.

  /**
   * Tests that event instances are updated when the event series is translated.
   */
  public function testEventInstanceUpdatedWhenEventSeriesTranslated(): void {
    // Initially the event series is in German.
    $this->assertSame('de', $this->eventSeries->language()->getId(), 'The event series is in German.');
    $this->assertEquals('Vortrag zum Thema Designinnovation', $this->eventSeries->label(), 'The title of the event series is in German.');

    $instances = $this->eventSeries->getInstances();
    $instance = reset($instances);
    $this->assertCount(1, $instance->getTranslationLanguages(), 'There is only a German translation of the event instance.');
    $this->assertSame('de', $instance->language()->getId(), 'The event instance is in German.');
    $this->assertEquals('Vortrag zum Thema Designinnovation', $instance->label(), 'The title of the event instance is in German.');

    // Translate the event series to French.
    $translation = $this->eventSeries->addTranslation('fr', ['title' => 'Conférence sur le design d\'innovation']);
    $translation->save();

    $this->assertEquals('Conférence sur le design d\'innovation', $translation->label(), 'The title of the translation is in French.');

    // Check the event instance is also translated.
    $instances = $this->eventSeries->getInstances();
    $instance = reset($instances);
    $this->assertCount(2, $instance->getTranslationLanguages(), 'There are now 2 translations of the event instance.');

    $french_event_instance = $instance->getTranslation('fr');
    $this->assertSame('fr', $french_event_instance->language()->getId(), 'The French event instance is in French.');
    $this->assertEquals('Conférence sur le design d\'innovation', $french_event_instance->label(), 'The title of the event instance is in French.');
}

The last line fails in this test, but passes when the patch from the MR is applied.