Originally, the app install ID was only transmitted as a query parameter. This was problematic for caching. For a while, we've been transmitting it both as a query parameter and in the X-Analytics headers, as a transition. We're now preparing to remove it from the query parameter, and only transmit the install ID via the X-Analytics header.
We'd like the apps uniques reports and session reports to be updated to use the X-Analytics header as well as the query parameter.
The reports will need to continue to support the old method using the query parameter because people that don't update their apps will continue to send their install ID that way.
The header documentation is here, under "wmf-uuid": https://wikitech.wikimedia.org/wiki/X-Analytics