Hi MrSharePoint,
Based on K2 5.1, when you created this role, did you check the "Refresh on Worklist" option?
http://help.k2.com/onlinehelp/k2five/userguide/5.1/default.htm#k2-management-site/user_management/roles.htm
When you add a new user to the role, the user receives existing worklist items assigned to the role when they log in or when the worklist refreshes. Select the check box if you want the K2 server to refresh worklist items based on the latest role membership.
If you don’t select the Refresh with Worklist option, any changes you make to the Role's membership only affects new worklist items.
NOTE: This option only applies to User Task steps where you do not check the Resolve Groups to individuals check box. When you check this option, the role is resolved to individual users as if you assigned the task to those users instead of the role, which causes the step to essentially not use the role or its Refresh with Worklist option. By default, Refresh with Worklist is unchecked because it does result in additional load on the server. You should only use this option if you have dynamic roles (such as in a call center scenario where users are continually rolling into and out of the queue) and your workflow tasks are assigned to the role without resolving the role to individual users.
-Jean