Updating INTranSplit from Custom Graph Throws "Source cannot be empty" and "Transaction Date cannot be empty"

Hi Team,

I am trying to update a custom field on the INTranSplit DAC from a custom graph. The update itself succeeds, but when I call Save.Press(), Acumatica throws the following validation errors:

Error: Updating 'IN Transaction Split' record raised at least one error.
Error: 'Source' cannot be empty.
Error: 'Transaction Date' cannot be empty.

The custom field (UsrPickedQty) exists on INTranSplit, and I am only updating that extension field.

public class TestGraph : PXGraph<TestGraph>
{
    public SelectFrom<INTranSplit>
        .InnerJoin<INTran>
            .On<INTranSplit.FK.Tran>
        .InnerJoin<INLocation>
            .On<INTranSplit.locationID.IsEqual<INLocation.locationID>
                .And<INTranSplit.siteID.IsEqual<INLocation.siteID>>>
        .Where<
            INTran.tranType.IsEqual<INTranType.assembly>
            .And<INTranSplit.lotSerialNbr.IsNotNull>>
        .OrderBy<INTranSplit.lineNbr.Asc>
        .View Picked;

    public PXAction<DummyDAC> UpdateSplit;

    [PXButton]
    [PXUIField(DisplayName = "Update Split")]
    protected virtual IEnumerable updateSplit(PXAdapter adapter)
    {
        foreach (INTranSplit split in Picked.Select().RowCast<INTranSplit>()
                     .Where(x => x.RefNbr == "000039"))
        {
            split.GetExtension<AKINTranSpliteExt>().UsrPickedQty = 2;

            Picked.Update(split);

            Save.Press();
        }

        return adapter.Get();
    }
}

What is the recommended way to update a custom field on INTranSplit and persist the change?

Version: 26R1