[Solved] Android App Notifications: vibrates but nothing else

I found both with Signal and also Conversations (XMPP client) that I'm not really getting notifications. The phone gently vibrates briefly, but the screen doesn't turn on and there is no sound. This includes for incoming calls in Signal: the phone briefly vibrates once, and if I switch it on I can see a notification for an incoming call, but this is extremely easy to miss.  I can't find anything wrong with my settings, but there are lots of settings spread through the stack. Is anyone else having trouble, and is there anything I should check?

Notifications of incoming mobile calls/SMS work fine, the screen turns on and it rings loudly and continuously. When there's an incoming XMPP message in the native Chat app, it makes a loud noise but the screen doesn't turn on. Not sure if that's how it's supposed to work.

Note that I am not interested in alternative native clients for these protocols.  I want Android notifications to work in general.  It seems that this problem exists for all Android apps.

Here are the various layers of settings I tried:

Conversations app, Settings / Notifications. "Message notification settings": "Importance, Sound, Vibrate". Tapping on that, "Default: May ring or vibrate based on phone settings", and "Sound: Default notification sound".

Android Settings app, Conversations, Notification settings. I have "All Conversations notifications" selected, "All 'Messages' notifications", "Allow notification dot".

Android Settings app, Notifications. "Bubbles: On / Conversations can appear as floating icons". "Do Not Disturb: Off".

Andromeda Android Settings: "Notification (Enable notification passthrough from guest to host" is enabled.

FuriOS Settings: Notifications. "Do Not Disturb" is disabled.