Okay,
I've been trying to recreate your situation and cannot -- neither using the negative Quick Receipt method (which, by the way, is the recommended way to return an item) nor following your steps. In both cases, as soon as the Inv/Adj is done, the receipt comes off of the O/S Receipts report.
Even when I have 2 separate items, each from different vendors, and I return one and adjust that vendors portion of the inv accordingly, both items are removed from the report.
Have you run a Data Integrity Check? Is it clean? If not, make sure you have a backup and do a Rebuild if the button presents itself.
Also, would you please verify the version and release date of PO you are using?
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Regards,
Softrak Tech Support