Custom map styling in react-native-maps not rendering, reverts to basic/standard map style

Working on a ReactNative App and I am trying to edit the map style.

I am following the instructions on "https://github.com/react-native-community/react-native-maps".

For IOS the docs suggest the below.

I.e:

1) Adding a mapStyle = [JSON being generated from https://mapstyle.withgoogle.com/].

2) Importing PROVIDER_GOOGLE

3) Adding 'customMapStyle' & 'provider' props (see below)

However, even though I have manually checked the JSON (namely that the colours are all black and grey, as per the custom map I want) The rendered map is still coming out in the standard blue and greens?

Oddly my markers are changing from pins to some android looking pins, so something is happening.

Any ideas welcome.

Thanks

import MapView, { PROVIDER_GOOGLE } from 'react-native-maps'

MapStyle = [ ... ]

render() {
  return (
    <MapView
      region={this.state.region}
      onRegionChange={this.onRegionChange}
      provider={PROVIDER_GOOGLE}
      customMapStyle={MapStyle}
    />
  );
}