Lookup ID field reverts to Zoho ID

I'm having trouble selecting a lookup field link other than the .ID when linking forms.

When creating the relationship in form-builder, I am presented with a choice for which field will be the link. In the past, I would select my own ID field (e.g Program.Program_ID) instead of the Zoho ID field (e.g. Program.ID). On these older forms, this definition show as I created it in the workflow->Form Definitions as well. ( values = Program.Program_ID )

Recently, I tried to create another form with a lookup to the same Program.Program_ID (not the Zoho ID). However it reverted to the Zoho ID. Even after editing in the Workflow->Form Definition, it would save, but then revert to the Zoho ID upon reload. ( values = Program.ID )

Has Zoho made it mandatory to only link to the Zoho ID in lookup fields? If so, why am I presented with a list to select lookup field when I create the relationship?