3816623 - Why some existing expense reports not associated with the default policy set in expense group?

We enabled 2 policies for one user group in expense section, one is default and the other not. Normally all user created reports should be auto assigned to the default policy since the policy field is set hidden for all in our entity. However, we found some expense reports not associated with the default policy.
Since users Cannot select policy, may I know why this happened?

Expense reports not associated with the default policy set in expense group , KBA , CON-CA-ADM-PAS , Policy Assistance , Problem