Multiple Instance: Add one variable simultaneously

I have started 5 external tasks in Multiple Instance mode (parallel). All external tasks add one named variable.
After completing tasks (got 200 {external-task-id}/complete on every task) only one of them have completed actually. Others 4 tasks have freezed (locked but not completed).

OptimisticLockingException should be appeared but not appeared (200 OK returned).
If I try to update existing variable - OptimisticLockingException will be appeared

What’s wrong?