When will the application receive a install_referrer with content utm_source=(not%20set)&utm_medium=(not%20set)?

The documentation states that it only occurs when you use the deeplink (http) to the play store:

For a deep link into Google Play where the referrer parameter is not present or empty, the referrer value provided is:

utm_source=(not%20set)&utm_medium=(not%20set)

For instance:

https://play.google.com/store/apps/details?id=com.google.android.apps.maps

Documentation here: https://developers.google.com/app-conversion-tracking/third-party-trackers/android