That isn't how it works. You didn't "use up" some of that keg. There is no "amount of beer to be used up" in them.
Consider the keg of beer like a game of Russian Roulette with 80 chambers and 1 bullet, and after every shot you spin the barrel.
If you fire a bullet, the game is over and a keg is taken away.
Even a "brand new, fresh keg" could vanish after just 1 beer.
There is no glitch here, you were just lucky that none of those 20 to 30 uses of the keg landed on the RNG "poof, it's gone" result.
Unless you already knew this of course, and your post was just meant to be humourous and failed.