In the ConfigEvents::SAVE event, the original data (to which overrides are applied) is compared to the current data (to which overrides aren't applied)

In the event subscriber, to check if the configuration object is changed, the following code is used.

if ($saved_config->getOriginal() !== $saved_config->getRawData()) {
  // …
}

Config::getOriginal() applies the overrides by default, while Config::getRawData() doesn't apply overrides. This means that the values returned from getOriginal() and getRawData() could be different, even if the configuration object didn't change.

The call to getOriginal() should pass FALSE as argument, to avoid comparing data with overrides with data without overrides.