Subchart configuration settings need to be at the top level, outside a global: block.

At a style level, each chart should be independently installable, whether or not it's used as a subchart. Something like the stable/mysql chart is a reasonable example: you can manually helm install mysql stable/mysql --set mysqlPassword=... without mentioning global. That means when you include it as a dependency its settings need to be under the subchart's key in the values.yaml file.

At a mechanical level, when the subchart is run, the subchartA settings are promoted up to be .Values and then the original global: is merged with that (see Subcharts and Globals). So the subchart itself needs to be aware of the difference

{{/* Option 1 */}}
{{ .Values.global.subchartA.enabled }}

{{/* Option 2 (within subchartA) */}}
{{ .Values.enabled }}

and at the top level you need to use the form that's compatible with the included chart.

(If you browse through the "stable" Helm chart repository you'll see global used fairly sparingly; rabbitmq allows you to declare global.imagePullSecrets but that's close to it.)