I don’t think this is a known bug. Apologies if it is.
I just tried a balance inquiry at an RBS cash point which showed an account balance of £0.00. I knew this wasn’t right so I requested a printed balance from the same machine which once again returned a balance of £0.00. I immediately received the notifications on my phone for both balance enquires.
I walked up the road to a Barclays cash point to try getting another balance inquiry which returned the correct amount the app was showing me.
Im not sure if this is an issue with RBS, Monzo or just some machines just don’t like prepaid cards