This is expected behavior.
ORA-39181 is caused by an unprivileged user who tries to export a table with a fine grain access control policiy applied. The table owner is subject to access control and may not be able to export all rows in the table. Only the rows that can be seen by that user are exported. In order to preserve integrity of the table, the user importing the table should have enough privilege to recreate the table with the security policies at import time.
To avoid this:
grant EXEMPT ACCESS POLICY to <exporting user>;