Truffle only unlocked permanently if you completed Pizza Pass Season 1. Otherwise, once you ran out of Truffle after the event was over, it disappeared.
It will likely reappear again in another event or through the story after Chapter 5 releases. If I recall correctly, when Pesto sauce was introduced in the Summer event a couple years ago, it was also something that only unlocked in that event, but it was later made available to everyone after Chapter 1. The same will likely happen to Truffle, so I'm sure it'll be back :)
