The overly-hard potato is a curiosity, all right. Fortunately this is a rare occurrence, though one I have heard about. A potato cooks and without the starch cells cooking out the water and softening as expected is definitely not the norm. Consider that potatoes are actually a living organism. As such, they are subject to some extremes in terms of performance and their cooking science within. I suspect in this case it was a concentration of high solids in the cells devoid of the usual water within. Fortunately, this is the exception and not the rule, and you can expect most potatoes to perform under normal circumstances as intended.

The opposite situation is usually the issue: Potatoes getting so soft they no longer hold their shape in a soup or stew. You may consider trying this- slice the raw potatoes into desired pieces/shapes and cook separately and close to the time when the stew is almost ready to serve. Start the potatoes in cold water and gradually increase the temperature to 130 to 140 degrees F for 20 minutes. The result is a chemical reaction called retrogradation – which helps to seal the outside edges so the cut potatoes hold their shape – gradually increase the temperature and finish cooking, then add to the stew. More potato chemistry information can be found at https://idahopotato.com/dr-potato/potato-chemistry-in-cooking