Thank you for your response, @fanfare!
I had a small gain and a larger carry-forward loss. The carry-forward loss was missed in the original filing. Once it was added to Form 8582 (Passive Activity Loss Limitations), it resulted in the calculation of a refund.
Schedule D was attached to 1040-X.
Line 6 (tax liability method) of 1040-X shows QDCGTW.
Line 8 seems irrelevant (Subtract line 7 from line 6).
