Hi and welcome.
I don't have any personal experience of metformin myself but I understand (Bilous and Donnelly, Handbook of Diabetes) that its actions are to increase insulin action (although they don't know how it does that) and lower glucose by reducing the ability of the liver to make and add glucose to the body.
You're reporting a drop from 84mmol/mol to 56mmol/mol. Over what time period? I guess a few things are possible - one or other of the A1cs might be wrong - it's rare, but it happens: or you've responded very well to the metformin. Or something else is going on. I think the other thing is that you've not had a firm diagnosis yet - for instance, you may not be T2.