Able to trashcan PO line item after GR/IR complete

We have a configuration that does not allow PO line item deletion if GR/IR is complete. This is true weather GR/IR matches the original PO or not. Bottom line, not supposed to be able to trashcan PO line item if any entry made in GR/IR.

But, one of my users was able to,...... that's the problem.

This particular PO had an item category of S - Third Party,...... I wonder if that would make a difference.