On two of my phones - topped up before due date (new total over $50) - got text message a few days later saying "time to top up" - ignored it because already topped up. Then next time I check there is $0.00 balance!
The four successive cs people I've talked with all agree that there's something odd there. Nobody at Vodafone seems to care very much. I think it's about to happen on the third phone - it just got a message saying "time to top up" so I guess it goes to zero in a day or so.
New policy at Vodafone?? Not just "top up OR lose your balance" - but now "Top up AND lose your balance!"