On 28 April, I posted on this trail that I was having similar issues with HomeKit automations, involving HomePod performing certain activities, disappearing from HomeKit shortly after they have been set up. I did more research into this issue on the internet. All seem to say to update the iOS devices to the latest version including for the HomePod. One suggested setting up a scene separately and then use HomeKit automation to call up that scene. Tried all these in the past week with no luck. Restarted the HomePod and AppleTV (4th gen) ... any iDevices which used the same Apple ID .... still no luck.
Then last night I tried setting up an automation ... not giving up yet. I set up separate scenes for the HomePod to play from various radio stations ... wondering when these scenes would disappear from my HomeKit. I set up an automation to start playing one of the radio stations for the next morning as a wake up alarm.
Surprisingly, I woke up to the HomePod playing that radio station this morning. And all the scenes that I have set up for other radio stations are still available on my iOS device's Home app.
I noticed something odd for a brief moment in one of my iOS devices in that it didn't have some of the scenes listed. It wasn't the device that I had set up those scenes with. Later, when I checked again, they had appeared. I wonder if that device got an update from my Apple ID.
Sorry for the long post. But just want to share my observations. So HomePod automations are now working for me after a week of frustrations. Not sure what the exact trick was to get it working. And not sure if the scenes and HomePod automations will hang around.