Item Supplier UOM for Substitute on AutoSource Issue occurring

The original item has two item supplier UOMs set up EA and BX with Box marked as default. On the Substitute item when the item supplier UOM is set up with just box the item does not substitute onto a PO. If I set up the substitute item with just EA or both EA & box it does successfully substitute. any idea what would cause this?

Tertiary info: both items have std UOM of EA & the conversion rate defined on the substitute is 1. Under the units of measure section of define items they have identical set up. PeopleSoft 9.2 PT 8.56.12 PUM 30. and for context the workflow is as follows, req is created from req loader, po auto source runs and performs the substitution automatically