Required setting not being saved back to Field Config entity when using the default (Core) Required API plugin.

This is a duplicate of https://www.drupal.org/node/2916137 → on required_by_role.

When selecting the default (Core) option for Requirement strategy, the requirement setting is not saved back to the Field Config entity, although the default plugin uses $field->isRequired() when checking whether the field should be required.

I've written a patch that does a hook_field_config_presave to check whether 'default' is selected, and then saving the required setting back onto the entity.