FCV upgrade to 7.3 (or 8.0) is supposed to set up a schema validator for config.settings (SERVER-82609). However, it is not working. The reason is that when the FCV command calls ShardingCatalogManager::upgradeDowngradeConfigSettings, the FCV version is still kUpgradingFrom70To73, so the 'gBalancerSettingsSchema' feature flag will still appear as disabled. (Note: The in-memory FCV is effectively transitioned later here, after which the feature flag will start appearing as enabled.)