Use of core.en-GB.resx file with core.resx file creates an issue like the resource key won’t get replaced by the resource string value.
You can either copy all the content into core.en-GB.resx or just removing the resx file with the current culture will resolve the issue.
For example in our Feature schema.xml we have the resource key $Resource:OpenMenuKeyAccessible. In runtime it won’t get replaced by the actual string so that a machine with low configuration just displays the Resource key as it is while loading the page. If you remove the resx file with your current culture then it will display the actual value or you need to add the string to the resx file with your current culture.