I just ran into a similar problem and I think I have a solution. I think it may be a Thunkable Height issue, rather than a problem with the Google API. I created an app to illustrate the problem.
https://x.thunkable.com/copy/4e2548820f847693d87746cf78efaad8
On the screen there are 3 columns, each with a map inside. The first column is set to height Fit Contents, the second is fill container, and the third is an absolute height.
When I view the app on my android device, the first column (fit contents) is completely collapsed (blue border). The other 2 columns are sized properly and show the associated maps.
I see in your app you have 2 columns. To debug the map issue, try setting those columns to an absolute height of something very small, like 1 or 10. Does the map then show up? If so, it may be an issue with the column sizing like I had.
Happy Thunking!