It looks like the objects in the response of the App Service are removed somehow.
I created this scenario. I have a datagrid with a App Service call to get the list of stories. The list renders just fine. When i select a story, the information doesn't show up in the data view that is listening to the grid. Also if you have a regular Edit button on the grid connected to a new/edit screen, nothing happens. When you create a microflow that uses the selection of the grid as a parameter, an error occurs, because the input parameter (selection) is empty.
Looking at the network traffic i don't see any reason why the client thinks those app service objects don't exist anymore, so personally i expect that objects resulting from an App Service are treated differently from normal transient objects.
It might work if your store the results from your App Service call in a transient object you defined yourself, but going from and to those objects seems like a lot of unnecessary work. I would probably file a ticket.