Only time I've had a BP disappear from the Android app was after the scheduled departure of a flight delayed by 3+ hours. I wrote them a bug report from inside the app about it. It was a bit of a hassle, as I was already through security but not yet in the lounge when it vanished.

Luckily, like most posters have said, I always get them to dump PDFs of the BPs into my inbox for later use (and Google Now reads them from there as well, which is handy).