For it to become unreliable would require upheavals in reality.
Or at least our conception of reality. But I think you’ve hit the nail on the head here. Math is so reliable that it’s unthinkable that it could ever change or lead us astray; and so most of us don’t think about it at all. But (and I can already feel eyes rolling) what are numbers? Why are they universally and necessarily true? Are we inventing new mathematical truths or discovering them? These are not questions for science.