Seems like there are many questions from users as to how Monzo do their research and launch products/features.
There is some useful insight in this thread from @SamanthaD that helps explain the rational behind how they launch and test new products plus how they do their research.
https://community.monzo.com/t/user-research-at-monzo/32235
In post 13 of that thread she explains a bit about why they dont tend to launch the full version straight away.