Partner Contact ID in Sales Order Entry

We use the power form version of sales order entry (P42101). There is a column on the entry grid named Partner Contact ID (PRCIDLN). We are seeing inconsistent behavior in our environments concerning this field, so I want to find out which is the expected behavior. In our production environment, if we key/select a value in this field during order entry, then later come back to zero/blank it out, that works fine. However, in our development and test environments, if you zero/blank this field's value and tab out, the original value 'bounces' back. You can never clear the field once