Mostly a note for others who might encounter the same issue.
Recently, restored a backup of a 6s to a XR. After the restore, tried to sign back in to Paperpile via my Google account. For some reason, this initial authentication failed (was redirected to the Google app page). Tried again, and the sign in appeared to succeed. Attempted to use Paperpile, but it crashed. Launched again, but it crashed after reaching the splash screen. Removed and reinstalled Paperpile. Never made it past the splash screen. Reinstalled TestFlight and Paperpile, and all seems well again.