ORA-02290: Check constraint (sys.ensure_json) violated

We are migrating database charaterset to uta8 using DMU, but came across this error.

The DMU process was trying to update the table with: update sys.modelalg$ set MDATA= SYS_OP_CSCONV(MDATA.'AL16UTF16'

Looking into the constraint, it's

ALTER TABLE SYS.MODELALG$ ADD (
CONSTRAINT ENSURE_JSON
CHECK (mdata IS JSON)
ENABLE VALIDATE);

strangely, not all databases have this table sys.modelalg$, and there is little info out there for this table either.

Please share your idea on this.

Database version: 19.25

server: rhel 7