Undefined array key "source" ConfigEventsSubscriber.php

I have tried to find a way to detect, if config is actually translation, but there is no methods for it.
The only way we can check it by storage collection. Or maybe there is another way to do it.
Anyway, I think in this case we just simply can check source and it would be enough

@LOBsTerr - I'm having trouble reproducing this issue.

Set up steps:

1. Install a vanilla Drupal 10.1.x site
2. Install oEmbed Providers module
3. Install Configuration Translation
4. Install Dutch language
5. Create a 'Test Media Type' media type
6. Add translation for media type configuration
7. Export entire site config to ../config/sync

If I drop the DB and re-install from existing config, I'm not seeing the warning.

I also tried setting a breakpoint in ConfigEventsSubscriber::configSave(). If I alter the translated ../config/sync/language/nl/media.type.test_media_type.yml file and import, the breakpoint doesn't hit. If alter ../config/sync/media.type.test_media_type.yml file and import, the breakpoint does hit.

So far as I can tell, ConfigEventsSubscriber::configSave() doesn't fire went translated config is imported.

I will double check it on clean installation and will come to you

I have followed almost exactly your steps, the only difference, I translated Image media type, instead of creating a custom one:
and this what I see during the installation

root@f91e89afa23e:/home/lob/www/clean-drupal# ./vendor/bin/drush 'site:install' minimal --root=/home/lob/www/clean-drupal/web/ --site-name='Clean Drupal website' --site-mail=info@example.org --locale=en --account-mail=admin@example.org --account-name=admin --account-pass=admin --sites-subdir=default --existing-config -y
 You are about to:
 * DROP all tables in your 'drupal' database.

 // Do you want to continue?: yes.                                                                                      

 [notice] Starting Drupal installation. This takes a while.
 [notice] Performed install task: install_select_language
 [notice] Performed install task: install_select_profile
 [notice] Performed install task: install_load_profile
 [notice] Performed install task: install_verify_requirements
 [notice] Performed install task: install_verify_database_ready
 [notice] Performed install task: install_base_system
 [notice] Performed install task: install_bootstrap_full
 [warning] Undefined array key "source" ConfigEventsSubscriber.php:68
 [warning] Undefined array key "source" ConfigEventsSubscriber.php:68
 [warning] Undefined array key "source" ConfigEventsSubscriber.php:68

If you want I can share my configuration. I also set breakpoints to discover it in the first place and I just did it again. The same result as reported, Are you sure your debugger setup to work with CLI ?

I'm able to reproduce the behavior when installing the site with Drush; however, it doesn't happen when the site is installed via UI. When installing with Drush, the breakpoint does hit, and I do see that 'source' is missing.

Additional info:
When running drush config:import to import either the media type or its translated config, ConfigEventsSubscriber::configSave() does not fire.

Diving deeper:

When installing via UI:
In ConfigImporter::ImportConfig(), the following line returns an overrider:
$overrider = $this->configManager->getConfigCollectionInfo()->getOverrideService($collection);
The config object loaded is Drupal\language\Config\LanguageCongigOverride, which has its own ->save() method, which dispatches a LanguageConfigOverrideCrudEvent event. The oEmbed Providers' ConfigEventsSubscriber subscriber is not triggered.

When installing via Drush:
In ConfigImporter::ImportConfig(), the following line does NOT return an overrider:
$overrider = $this->configManager->getConfigCollectionInfo()->getOverrideService($collection);
The config object loaded is Drupal\Core\Config\Config. When its ->save() is called, it dispatches a ConfigCrudEvent event, which triggers the oEmbed Providers' ConfigEventsSubscriber subscriber.

In ConfigImporter::ImportConfig(), when installing via UI, $this->configManager->getConfigCollectionInfo() returns two collections: language.en and langauge.nl. When installing via Drush, it returns only language.en.

I spent some time digging into this, and it really gets into the weeds of the Drupal installer and how Drush invokes it. This is being caused by either a bug in core or in Drush.

This module extends the provider-side of core's oEmbed functionality. If an issue does exist, it's a core issue. Please feel free to open an issue against Drupal core.