Big issue with hydrogen fuel cells is you need large amounts of platinum group metals per vehicle, and not just in a flash coat a few atoms thick like in catalytic converters, but in measurable thicknesses. As well they are exposed to atmosphere deliberately and thus are susceptible to poisoning by contaminants, like lead ( lead free fuel is not totally lead free, simply because of all the older equipment that had decades of TEL in them leaving a coat, plus the base oil is not exactly lead and heavy metal free anyway) and just degrades with time anyway. 1Troy ounce of platinum and Rhodium per vehicle does add up rather fast, and it is a cost.
Hydrogen generation in lead acid batteries is mostly in charging, and most modern batteries have added things like antimony in the plates to recombine this to not lose water, and for larger flooded batteries there are also catalytic recombiners that reduce the venting of Hydrogen and also reduce water loss in the cells. just sitting idle they do not generate gas, while stored high pressure hydrogen, or a metal hydride, does always have gas diifusing out through the grain boundaries of the material, nothing is going to stop hydrogen, just slow it down, other than cryogenic freezing it to a slush and keeping that in a really good Dewar flask tank. 2Kelvin is not exactly something easy to achieve, despite the best attempts by space companies, as this gives a 10% improvement in fuel density over the regular liquid as it is denser.