I did some patching with mortar mix, which I've done dozens of times before but this time after it cured the patches just turned to powder and rub out with my finger. Is the mortar mix bad? What if it's been sitting around for a while?

In old dry mix concrete, quik crete e.g. the lime, calcium oxide, absorbs water and carbon dioxide from the air and becomes calcium carbonate. No bonding power. This will also happen if you let the concrete dry out too fast. You can't fix it without starting over with some new material. If you are patching, use the vinyl patcher.