Tim advice is very important. You need to make sure there alter session statement is included in your pre/post script.
Unfortunately Masking Engine is removing a comments from SQL so as Oracle hints are enclosed by begin/end comment - there are removed as well.
You need to set parallel level on object and then Oracle CBO will execute this using a parallel mode, at the end you should reverse a change the value which was set on table before. As it executed in non parallel mode I assume it was set to non parallel.
You script should looks like this:
ALTER SESSION ENABLE PARALLEL DML;
ALTER TABLE TABLE_NAME PARALLEL 64;
UPDATE TABLE_NAME SET UPDATED_COL =UPDATE_COL where CONDITION ;
ALTER TABLE TABLE_NAME NOPARALLEL;