Can you create a small example application that allows to reproduce your issue?

I confirm this issue, and the reason is that $container->import() appends the config instead of prepending it, making it the preferred config in the stack, even over the app config (which is loaded earlier).

That's why we recently discouraged using $container->import() in prependExtension symfony/symfony-docs#19739.

But don't be sad 😄, you can use it starting with Symfony 7.1 #52843. Meanwhile, for previous Symfony versions, I've backported that feature into this package https://github.com/yceruto/micro-symfony, which fixes this issue. Otherwise, you can always use $builder->prependExtensionConfig() instead, with the actual limitations.

Related to #52789

Closing as this is an expected limitation prior to 7.1 and fixed in 7.1. Thanks for reporting!

you can always use $builder->prependExtensionConfig() instead

@yceruto @xabbuh It seems to me that this method is also a bug. When trying to add a field that is not yet in the application configuration file (../configs/liip_imagine.yaml), it produces an error like

Warning: Undefined array key "resolvers"

    public function prependExtension(ContainerConfigurator $container, ContainerBuilder $builder): void
    {
        $builder->prependExtensionConfig('liip_imagine', [
            'resolvers' => //...

../configs/liip_imagine.yaml

liip_imagine: ~

Can you create a small example application that allows to reproduce your issue?