Burned by this a bit today, but actually in the opposite way in that my seat was given to a standby without voucher compensation (initially).
Was #1 on the UG list for CLT-PHL, volunteered for later flight. Checked with agent midway through boarding to confirm volunteers were still needed and was told to wait. Group 9 came and went and my upgrade was confirmed on list then given to next pax. Stepped to the podium following boarding to discuss voucher and agent looked startled and said that volunteers ended up not being needed. Apparently volunteering placed me in some sort of AA IT purgotory in which my seat and upgrade got taken, but I was listed as simply missing the flight. Agent suggested I not volunteer anymore. Any insights as to how this happens?