Warning - Some apps haven't been configured to use Apple's SKAdNetwork

Today I’ve noted this notification on my Admob console:

** Some apps haven’t been configured to use Apple’s SKAdNetwork*

To ensure you’re getting credit for all ads activity, like app installs, be sure to configure SKAdNetwork with Google’s recommended network IDs.

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I don’t know why I’m receiving this warning if my app in App Store uses Admob and contain the SKAdNetworkItems key in build.settings, which looks like this:

GADApplicationIdentifier = "myAdmobAppID",
          SKAdNetworkItems = {
                  SKAdNetworkIdentifier = "cstr6suwn9.skadnetwork",