Does anyone with these problems also happen to run Avast antivirus? I just found this topic: https://forum.avast.com/index.php?topic=222282.0 with very little info so far.
The link is spot on!
Yes, I have Avast and it was on Silent Mode and yes I had "Windows Push Notifications User Service_193fbd92" turned off. Once I went to "services.msc" and turned on the service, Focus Assist started to work. No restart required.
I have no idea why the name looks so weird. The description of the service is "This service hosts Windows notification platform which provides support for local and push notifications. Supported notifications are tile, toast and raw." There is another service called Windows Push Notification System Service, which was running.
I was able to reproduce the problem by stopping the notifications user service and then going to Focus Assist in Settings and it crashes. I don't know if Avast is turning the service off, but surely it's a bug in Windows to not handle the service being off in Settings.