@xabbuh Even if it's not called this need to be fixed, but as speaking with @nicolas-grekas, we found out that there are something strange in here:

• ConfigurationWithArrayNodeRequiringOneElement::getConfigTreeBuilder is never called
• EnvExtension::load has an

if(!array_filter($config)) {return;}

and in the test using ConfigurationWithArrayNodeRequiringOneElement, the $config array is empty.
So maybe we have more to do in here than just fixing the merge. (we have the same issue with EnvConfigurationWithoutRootNode)