I often try and pay my Barclay card bill prior to the direct debit as I only use the card for work expenses. So when work pay me, I pay the card.
The Barclay card app is pretty basic and seems to detect the Monzo CA card as a Maestro Card. It forces you to enter either an issue number or start date. I just entered the date I got the card as the start date and it was all accepted fine. TLDR; it works but you have to lie to it.