[Mondial]

Those tubes run very hot. The glass envelope is very small and the plate structure is very close to it. Even the filament itself dissipates about 7.5 watts, so there is a lot of heat generated.

If you look at the schematic, the grid resistances are in the several megohm range, so it takes very little gas current to upset the balance. In this case, the circuit is far more sensitive to gas than the tester. These tubes were designed for TV horizontal output use where gas has little effect, so possibly the tester does not test them too critically.