Even though the extId is uniquely generated, internally when we query children, we calculate the keys based on the primary keys for that table. In case of multiple columns within the primary key, we append them, so for child table both of the following entries end up having the same value,
10000020875038
10000020875038
Workaround :Change one of the values or auto incremental the column to the primary key.
Permanent Fix:BE Engineering confirmed the reported issue is a bug and BE defect (BE-24841) and will be fixed in BE 5.3.0 HF3