If you set the a setting in a parent theme that is not overridden in the child theme and that setting is used by the 'theme_boost_get_extra_scss' Boost callback which has been invoked by the method 'get_pre_scss_code' of the 'theme_config' class in '/lib/outputlib.php' then it is not used by the child theme because the SCSS is generated with the settings of the child and not the parent.  This is down to two reasons:

• 'get_pre_scss_code' calls the function 'theme_boost_get_extra_scss' in Boost's lib.php via the '$THEME->prescsscallback' setting in Boost's config.php file using the 'theme_config' object of the child and not the parent.
• and if that is rectified, then the 'parent_configs' -> 'setttings' attribute has the settings of the child theme and not the actual theme that the 'parent_config' (theme_config object) belongs to.