Installation of app from Briar third party repo asks for allowing "unknown sources" despite the privileged extension being installed

When I try to install Briar from their third party repo (https://briarproject.org/fdroid/repo) I get prompted to allow unknown sources by android, although I have the privileged extension installed and working with the official f-droid and other third party repos. Other users have also experienced this with the Briar repo. Android Version: LineageOS 14.1, based on Android 7.1.2 Device Model: BQ Aquaris U Plus F-Droid Version: 1.4 ADB-Logcat:

10-13 21:35:24.226  8242  9267 I ActivityManager: START u0 {act=org.fdroid.fdroid.installer.DefaultInstaller.action.INSTALL_PACKAGE dat=content://org.fdroid.fdroid.installer.ApkFileProvider/files/Briar-1.1.1.apk cmp=org.fdroid.fdroid/.installer.DefaultInstallerActivity (has extras)} from uid 10067 on display 0
10-13 21:35:24.227  8242  9267 W ActivityManager: startActivity called from non-Activity context; forcing Intent.FLAG_ACTIVITY_NEW_TASK for: Intent { act=org.fdroid.fdroid.installer.DefaultInstaller.action.INSTALL_PACKAGE dat=content://org.fdroid.fdroid.installer.ApkFileProvider/files/Briar-1.1.1.apk cmp=org.fdroid.fdroid/.installer.DefaultInstallerActivity (has extras) }
10-13 21:35:24.446  8242  8555 I ActivityManager: START u0 {act=android.intent.action.INSTALL_PACKAGE dat=content://org.fdroid.fdroid.installer.ApkFileProvider/files/Briar-1.1.1.apk flg=0x1 cmp=com.android.packageinstaller/.PackageInstallerActivity (has extras)} from uid 10067 on display 0
10-13 21:35:24.606  8242  8298 I ActivityManager: Displayed com.android.packageinstaller/.PackageInstallerActivity: +136ms (total +187ms)
10-13 21:35:25.317  8548  8565 I art     : Background partial concurrent mark sweep GC freed 161316(10MB) AllocSpace objects, 24(1212KB) LOS objects, 38% free, 25MB/41MB, paused 1.849ms total 200.507ms
10-13 21:35:31.453 32109 32109 I PackageInstaller: Finishing off activity so that user can navigate to settings manually

The log starts when I tap the "Install" button in F-Droid after the .apk has already been downloaded. The last line of the logcat only appears if i hit "cancel" in the "allow unknown sources" dialog. Otherwise it waits for input.