Yes we've had that too. Really annoying if your phone is in another room with your children sleeping in the room next ot it.
It came back twice. I think it has to do with Signal messenger update notifications. I had been looking all over the phone but it only stopped when tipping those notifications. They seem to be classified wrong. Normal Signal notifications don't do that.

And Signal stuff (or android notifiactions in general? ) seems to get not their specific icon but a "exclamation mark on blue button" with XA2. The X is not doing that, both icon and alarm.