How to forward a task by a user who belongs to a group in BPMS

Issue

• I am getting a PermissionDeniedException when trying to forward a human task. My task is assigned to a group "group1". "group1" has "user1" and "user2". After started the process, this task is in a 'Ready' State, and potential user1 forwards the task to potential user2, but it throws a PermissionDeniedException.

caused by: org.jbpm.services.task.exception.PermissionDeniedException: User '['UserImpl:'user1']' does not have permission to execution operation 'Forward' on task id 391
at org.jbpm.services.task.internals.lifecycle.MVELLifeCycleManager.evalCommand(MVELLifeCycleManager.java:98)
at org.jbpm.services.task.internals.lifecycle.MVELLifeCycleManager.taskOperation (MVELLifeCycleManager.java:322)
at org.jbpm.services.task.identity.UserGroupLifeCycleManagerDecorator.taskOperation(UserGroupLifeCycleManagerDecorator.java:460
at org.jbpm.services.task.impl.TaskInstanceServiceImpl.forward(TaskInstanceServiceImpl.java:166)
at org.jbpm.service.task.commands.ForwardTaskCommand.execute(ForwardTaskCommand.java:65)
at org.jbpm.service.task.commands.ForwardTaskCommand.execute(ForwardTaskCommand.java:45)
at org.jbpm.services.task.commands.TaskCommandExecutorImpl$SelfExecutionCommandService.execute(TaskCommandExecutorImpl.java:65)
at org.drools.core.command.impl.AbstractInterceptor.executeNext(AbstractInterceptor.java:41)
at org.jbpm.services.task.persistence.TaskTransactionInterceptor.execute(TaskTransactionInterceptor.java:54)
at org.jbpm.services.task.commands.TaskCommandExecutorImpl.execute(TaskCommandExecuteImpl.java:40)
at org.jbpm.services.task.impl.command.CommandBasedTaskService.forward(CommandBassedTaskService.java:168)

• After the task is forwarded, when I "user1" query for getTasksAssignedAsPotentialOwner I should not see task return to me. But, I still this task return to me.