I would probably reach out to customer support to see what might be happening since it seems like everything you are doing is right. Might be something missing that I am unaware of.
When the Snapshot function call is encountered in the email or create file template, the AppSheet server invokes the Google WRS web service that is capable of capturing the contents of a view to an image file.
The AppSheet server passes the WRS web service a special token that allows WRS to invoke the view you specified in the Snapshot function.
The WRS web service, invokes your application using the special token, your application displays the requested chart or graph view, WRS captures the view contents to an image file, and WRS closes your application.
The WRS web service returns the captured image file to the AppSheet server.
The AppSheet server inserts the captured image in the PDF file.
At present, when WRS invokes your application, the UserSettings are not passed to your application. As a result, Snapshot may not work properly if your application relies on the UserSetting being present for the view to be displayed correctly. For example, security filters or view filters that refer to UserSettings will not work properly.