Actually, I was having the same problem with Brave on mobile; I could not install Silverbullet as a PWA.
Turns out this was caused by Authentik. After adding the following paths (which I took from the Authelia documentation page) to “Unauthenticated paths” in Authentik I can now install Silverbullet, just like other apps!
^/.client/manifest.json$
^/.client/[a-zA-Z0-9_-]+.png$
^/service_worker.js$