The system is more or less as you speak (I’m in a common law country) but I’m speaking of specific instances.
That if conviction has already been received (proven beyond reasonable doubt), and that there will be an appeal of the accused. Onus of proof will be on the accused to prove his innocence in his appeal.
I was specifically thinking of a free civilian who was falsely imprisoned, as in a tort. It technically would be a false imprisonment on an innocent in gaol (if he’s been proven so), but that’s ground would unlikely be used.
That upon their conviction is quashed, a former accused that is still in prison, would be able to have an administrative appeal on ultra vires. I can’t think of any instant anyway, where a quashed conviction would keep a person in gaol.