Facing OptimisticLockingException while calling multiple call activities

I am trying to call a generic call activity multiple times in a flow.
But getting OptimisticLockingException. ENGINE-03005 Execution of ‘DELETE TaskEntity’.
Have attached a snippet of the BPMNs.

Generic_Demand_flow.bpmn (9.8 KB) Generic_UserTask.bpmn (2.3 KB)

I am using oracle sql as a database.
Using 12c dialect.