Merge issue in unidirectional OneToMany relation

This is a barebone Symfony project in order to isolate and help explain this issue.

How to reproduce:

1. Checkout code
2. Setup a MySQL database
3. Create a .env file based on .env.dist and configure DATABASE_URL environmental variable
4. Install dependencies: composer install
5. Run migrations: php bin/console doctrine:migrations:migrate
6. Load fixtures: php bin/console doctrine:fixtures:load
7. Start web server: php bin/console server:run
8. navigate to home route (default: http://127.0.0.1:8000/home)

Expected behaviour

I expect Post 1 to be removed from the post table.

Actual behaviour

Post 1 still exists in the post table.