Android Enterprise mandatory apps can be uninstalled
From your screenshot, your Gboard app defined in the app-catalog rule is from Google Play Store rather than Managed Google Play store. So, it is not a managed app, which means that it can be uninstalled by device end-user even if the AE feature-control option "Disable Managed App Uninstallation" has been checked and deployed.
When the device is Administrator mode you can uninstall the applications, and the applications cannot be uninstalled when the device is in User mode.
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Have you deployed any Android-Enterprise feature control profile with the option "Disable Managed App Uninstallation" checked?