At the moment, there is no indication from Pega on when or if JSON automapping for page groups will be supported in the future. However, there are some workarounds you can try to get the desired JSON output.
One workaround is to use a data transform to convert the page group into a JSON string manually. You can use the "Java Object to JSON" method available in Pega's expression builder to convert the page group to a JSON string. This should give you the desired JSON output with proper data types.
Another workaround is to create a custom function that serializes the page group to JSON. You can use a third-party library such as Jackson or Gson to serialize the page group to JSON with proper data types. Then, you can call this custom function from your code to get the desired JSON output.
Both of these workarounds require some custom development work, but they should give you the desired JSON output with proper data types.
I run into the same problem with a page group not transformed to JSON. The workaround I found is quite simple. I created a single page property into which I added the page properties as used in the page group. Then I use a data transform to copy the pages. In the JSON data transform I am adjusting the output name and got the result I need. I am running the data transform to map the pages from the page group to the single page in the activity before I call the data transform to serialize the JSON: