Why did £300 disappear from my John Lewis gift card?

I’m wondering if it’s safe to use such cards if the balance can just go missingWe do our weekly shop at Waitrose using a John Lewis/Waitrose gift card. I am a teacher and my union offers a 6% discount on topping it up. However, this week the balance mysteriously dropped by £300. Where had my money gone? After initially being advised to report it to the police as fraud, we were told it was a glitch with the gift card system. After much effort on our part, it has refunded the mon