It seems much more likely that charges moving from pending to posted would be good news, as opposed to disappearing completely. Disappearing charges might show up again directly as posted or they might just be gone until AT&T has more stock.
No idea why having the pending charges fall off would imply anything positive.
They fall off after three days. It's when they come back on, that's the good news. Like for me, charges disappeared at Midnight and my card was re charged at 4am.