As I've been informed it's not recommended to use these out of box stored procedures like TASKGROUPFORWARD or TASKOFFICERFORWARD to make task assignments through direct database modifications. It does not work for the same reason of cached data. Although you still can make database updates for custom (extension) fields as it seems they are not cached the same way as the main entity data.
Besides after a prompt look into your decription it seems that you don't need to update the database to make a task assignment. If you have an extension field specifying the group you can easily make a task assignment to that group. Simply you should assign the task to "Multiple Recipients/Field" and add the custom extension field to the task recipients list.