Hey @NickTu @Bart_Elia
Don’t know where you ever got to with this issue but with new eyes and a better understanding of epicor implementation. I have found why this is an issue…
How Epicor works (sometimes) is that apparently it wants two datasets for the same transaction. In this case the PlantTran which is what we are trying to update requires a copy of the original datarow, followed by the same datarow with the modifications that have been made. This is not reflected in the REST API or the Traces you do in epicor. The REST implementation has issues with this in a number of other places (one off the top of my head is CustShipSvc.CustomUpdate ). Apparently we just have to figure this stuff out 
I think this is because Epicor in some custom methods checks the original dataset to the modifed dataset in order to determine what values have changed, not 100% on this though.
So if you want to see a transaction that worked for me in action in my test system I created a transfer order receipt with 3 lines.
I did the GetByID and modified the save behaviour so that it duplicates each row and on the second row it sets the RecTranDate = today, RowMod = ‘U’.
When i submitted this to CustomUpdate() i had effectively double the rows. The method ran successfully…
Please see attached JSON in text(look at PlantTran dataset) to see what i am talking about.
TransferOrderReceipt.txt (35.2 KB)
Hopefully this made sense