Hi Julie,
There is a way to work around this.
You will need to create a queue and name it something like 'System Callback Queue' and ensure there are no agent/members assigned to it. This is just so you know what the queue is used.
Create an in-queue flow, that has 'disconnect' action only and assign it to the queue created earlier.
When caller accepts the callback and the callback is placed in the queue, instead of simply disconnecting that customer call, you can transfer it to that 'System Callback Queue' queue, where the call will be disconnected as soon as it gets there.
This way, actual callback requests won't be treated as abandoned calls on the real queue, but will be in the System Callback Queue.
Hope this helps,
Kirill.