21R1: GI Data Field Relations do not allow you to join fields from other tables in the GI

In a GI, I need to query the WarehouseDetails settings for Material items included in MFG BOMs.

Normally I would join AMBomMatl to INItemSite ON AMBomMatl.InventoryID = INItemSite.InventoryID AND AMBomMatl.SiteID = INItemSite.SiteID.

In this case however, the client has not populated AMBomMatl.SiteID, so I thought I could add AMBomItem to the joins and use AMBomItem.SiteID = INItemSite.SiteID instead.

In SQL, the query would look like this:

SELECT * from AMBomItem
JOIN AMBomOper
    ON AMBomItem.BOMID = AMBomOper.BOMID and AMBomItem.RevisionID = AMBomOper.RevisionID
JOIN AMBomMatl
    ON AMBomMatl.BOMID = AMBomOper.BOMID AND AMBomMatl.RevisionID = AMBomOper.RevisionID
    AND AMBomMatl.OperationID = AMBomOper.OperationID
LEFT JOIN INItemSite
    ON AMBomMatl.InventoryID = INItemSite.InventoryID
    AND AMBomItem.SiteID = INItemSite.SiteID

Prior to 21R1, the GI Relations allow me to join any field from any of the tables included in the GI, but in 21.106.0024 , I cannot join fields from AMBomItem to INItemSite.

I attached the GI XML.

Could this be related to pre-defined settings in the DAC Relations?

Thanks in advance for taking a look.

 PS: I can populate AMBOMMatl.SiteID in order to solve the data problem, but I really would like to know the reason why a given DAC is not available for joins in a GI.