Oracle 19c. REF Partitioning. Start redefinition Holds TM lock on parent table in 4 mode (when insert data into interim table)

Question

I have this TM lock on interim's parent table for the whole duration of loading the interim table of redefinition (DBMS_REDEFINITION.START_REDEF_TABLE) that has a FK to a table used by other session that insert / update data ( requesting TM in 3 mode )

I was expecting the redefinition will be 'online' without blocking other sessions, but apparently this FK ( I guess ) breaks the whole plan.

Parallel DML&DDL is used.

Any clue on why and if there is a workaround/fix ?

Thanks