You need to make a formal complaint to Monzo. I recognise ‘need’ may seem a bit strong, but given you’re saying you’ve had problems twice now, that takes it right past the ‘should’ stage, I’d say.
Complaining directly to Monzo is the best - perhaps only - way you can get them to take the issue seriously, and I think you’d be unlucky if they didn’t give you a token amount as compensation (£20 odd).
ETA: have a read of the most recent posts in this topic: Getting limits increased