May be you can try this
Login to Workspace as an Administrator and under the Administration tab, edit the assignment for that Role and add the new group name
Logout and relogin as the user and see if the tasks show up
It worked, We should add a group in Administrator tab as you mentioned
This problem with the deployment. First time when you create role and add group or user into this in Jdeveloper and deployed it on SOA server. It creates that app roles and assigned that group /user to that role. That's why it worked perfectly fine.
But second time when you change group/user in process role and redeployed on soa server it would not reflect. You have to manually add that group in that role.
So thumb role is on each redeployment if you changing group/user in process role then change it manually also on bpm workspace.