Thank you for that link, @Mattches . I checked it out and found that in Settings, Notifications & Actions, under ‘Get notifications from these apps’, Focus assist (via Cortana) was turned off and greyed out. Further down Cortana was turned off, as I prefer it that way. But when I turned it on, it then allowed me to turn on Focus assist above it. I could then go back to Cortana and turn it off but now Focus assist (via Cortana) remains on, but greyed out.
Hopefully this is now the permanent solution to the issue and also explains why Focus assist kept auto-reverting to Alarms only.
I’ll monitor this over the next couple of days and see if it has cured the issue going forward.
PS - I had previously tried setting Focus assist to Priority only, instead of Off, and customised it to show notifications from Brave. However that did not work either, but perhaps it will now that I’ve done the above fix.