Symptoms

Get Least Work TaskAllocationRole method is only assigning tasks to the first user, when three users are in the role. It does this regardless of how many tasks the user has.
 

Diagnoses

In looking at the logs, the error "K2:2DOMAINUSER] does not have permissions to impersonate user K2:2User in role]" is displayed for every user in the role.

The TaskAllocationRole SmartObject runs only under the K2 Service Account (which is whatever account is running the K2 service). The account running the K2 service needs to have the Impersonate K2 server right.
 

Resolution

Grant the account running the K2 service Impersonate rights in Workspace > Management Console > Workflow Server > Server Rights

If the account in the error is the logged-in user's account and you are running K2 in Console mode, please make sure to start the Console as the K2 service account as opposed to running it as the logged-in user.