But in many trials (e.g. in pre-Conquest England), innocence was not determined by a miraculous absence of injury, but whether the inevitable wounds were healing or festering after a certain number of days. That’s a lot harder to pre-arrange.
And if everyone who chose to undertake the trial did so because they were innocent, then the fact that “nearly two-thirds of defendants were unscathed by the ‘red-hot’ irons they carried and hence exonerated” in early 13th century Hungary actually reveals a wrongful conviction rate in excess of 33%.
Plus religious scepticism was a thing in the Middle Ages: see e.g. this article (though admittedly that does deal with a period long after peak popularity of trial by ordeal). It doesn’t follow that every innocent person would expect God to protect them.
