expo: [expo-MapView] Can't get custom map style to work in Expo app

🐛 Bug Report

Can’t get custom map styling to work for my Expo iOS app. Example code below

Environment

Expo CLI 3.13.1 environment info:
    System:
      OS: macOS 10.15
      Shell: 5.7.1 - /bin/zsh
    Binaries:
      Node: 10.15.1 - /usr/local/bin/node
      Yarn: 1.13.0 - /usr/local/bin/yarn
      npm: 6.9.0 - /usr/local/bin/npm
      Watchman: 4.9.0 - /usr/local/bin/watchman
    IDEs:
      Xcode: 11.2/11B52 - /usr/bin/xcodebuild
    npmPackages:
      expo: ^36.0.0 => 36.0.2 
      react: 16.9.0 => 16.9.0 
      react-native: https://github.com/expo/react-native/archive/sdk-36.0.1.tar.gz => 0.61.4 
    npmGlobalPackages:
      expo-cli: 3.13.1

app target: iOS TestFlight app

Steps to Reproduce

In the Map component:

import MapView, { PROVIDER_GOOGLE } from "react-native-maps" 
export const Map = () => {  
  return (  
    <MapView  
      customMapStyle={mapStyle}  
      provider={PROVIDER_GOOGLE}  
    />  
  )  
}
const mapStyle = [ ...generated JSON from https://mapstyle.withgoogle.com  ]

and in my app.json:

  "expo": {
    ....
    "ios": {
      ....
     "config": {
       "googleMapsApiKey": MY_API_KEY,
      }
    },
  }
}

Expected Behavior

I’ve made sure i’ve enabled the Google Maps SDK for iOS service in the developer console, and have an unrestricted API key that allows both these services.

Actual Behavior

Result is the Google Maps map shows but without the custom styling

Reproducible Demo

Ask me for the testflight link. Anybody know anything about this? Haven’t yet managed to test on Android so i’m not sure if it’s something to do with iOS, or both, (or just something wrong in my code)