I've got an older Bounder that uses 2-2A1 and 2-1A1 'sealed beam' lamps and I've had the same problem for like 10 years now with my supposedly, laughingly, Sealed Beam called headlights. I've used GE, Sylvania, and Philips and all have developed leaks.
Since I seldom drive at night, and since the moisture, as mentioned above, eats the reflective coating off the inside of the lamp, but the bulb still works, I just wrap the outside with aluminum foil and liberally apply tape to hold it. Not as bright, but they work.
I don't drive much at night anyway. Doubt if the manufacturers care if they fail early because of leakage.
