I have the problem last time when I do with Apple Pay linked Halifax debit card top up. Finally used debit card directly.
I canāt get Apple Pay to top up my account at all. Mondo says itās topped up but at the same time I get a notification saying declined. The money doesnāt appear in Mondo at all but also no notification in mondo that the transaction declined. The topup works fine using debit card method. 
The Lloyds Banking group are bad at flagging these transactions as possible fraud. Iāve had issues with Lloyds but Iāve seen posts from others who use RBS or TSB.
You may need to talk to their fraud teams.
But that top up error from the Mondo app looks like a maybe bug.
Iāll try a smaller amount next time I top up and see how that goes, possibly declined due to that I havenāt phoned the bank to see if it declined at their side or not
