Symfony4 Error loading classes custom folder "Expected to find class... but it was not found"

Update your composer.json autoload setup

{
    [...]
    "autoload": {
        "psr-4": {
            "TestNamespace\\": "TestNamespace/",
            "": "src/"
        }
    },
    [...]
}

After run: composer dump-autoload and try again.

composer dump-autoload --classmap-authoritative will only work if your src directory is present at the time you run the command.

This can be an issue with multi-stage Docker builds in particular, when you are normally only copying the composer.json/composer.lock into the build image.