Whether MWG looks at embedded URLs is controlled under Settings -> URL Filter -> Default in the section Ratings Settings. The second option is Search for and rate embedded URLs. When deselected, the URL that you listed above will show up as Public Information only.
Yes, subdomains might add other categories to an original domain's category.
If you are having problem with this, you could just have a domain whitelist by creating a URL.Host.BelongstoDomains parameter and add maps.google.de to the list you'll create with the rule.
Put this on top of the blocking rule with a 'stop rule set' action.