I'd think it shouldn't be, also; however it sure could be. Some tubes were actually designed for extremely long storage and operating life because they were intended to be used in places that were completely unaccessible for service, like those used as repeaters in the original Transatlantic cable. Special materials and techniques. I do have receiving tubes from the 1950s still working fine, so there's no question that some tubes last a really long time without specifically being designed for that. I don't know much about 1625s and the only ones I ever saw on the market, even in the 60s, were all WW2 surplus. I've never been involved in the vacuum (or gas) tube manufacturing industry, but did "run" a thickfilm power hybrid operation for a couple of years in the late 1980s and the hermetic packages also used preassembled Kovar pin seals with glass or ceramic insulation, kind of like what's used with tubes. Usually larger pins, as these had to handle serious current (often as much as 50A), but still the same materials, and some lower power hybrids running at only 1-2A used small pins. We had to gas leak test them 100% after they were assembled in a nitrogen environment (not a vacuum) and had plenty of rejects due to bad Kovar-glass seals. That's where they leaked, and if they did of course were rejected and never shipped.