Lockscreen notifications setting doesn't behave as described
On Android versions lower than 8, we have a setting called "Lock Screen: Show notifications on the lock screen", which is disabled by default. The description suggests that if the setting is enabled, notifications will be shown on the lockscreen, otherwise they won't. But this isn't how it behaves.
When the system-wide setting is set to "Show all notification content", Briar's notifications are always shown, regardless of the Briar setting. When the system-wide setting is set to "Don't show notifications at all", Briar's notifications are never shown, regardless of the Briar setting.
When the system-wide setting is set to "Hide sensitive notification content", Briar's notifications are always shown, and the Briar setting controls whether they contain content (e.g. "Briar: New private message") or not (e.g. "Briar: Contents hidden"). the behaviour depends on Briar's setting: if it's enabled, notifications are shown with their contents hidden, otherwise they're not shown.
Since the setting has only a minor impact, the simplest solution might be to remove it.