Acropora frags encrust first prior to growing outwards to first form a secure base and strong foothold. Once the frag plug is mostly/fully encrusted, you will then see a spurt of growth outwards.
Sometimes both happen simultaneously also like in my case with my tricolor acro.
I would say that montipora is growing above average/fast rate. Great healing on it and to both heal and grow within 2 months should be a great sign of things to come for it.