Attempting to reduce Open Quantity on PO; Inconsistent Results

Hi,

We are on 9.2

I wondered if anyone can explain why the inconsistencies for the following:

A user was attempting to reduce the quantity on an open PO in production and received the below error messages. 

The PO contains prior receipts against it and some of the receipts have been vouchered.

 I replicated in PY, however, in testing (2) different PO's, I received inconsistent results.

Both of my PO lines are at NS 400 have line type 'S', both have an open balance, and both have prior receipts receipts vouchered. 

When I reduced the open quantity on PO A, tabbed down, no error received and qty was updated.