I think I may have stumbled upon the trigger that knocks it off of homekit.
I’ve been emailing back and forth with a tech from rachio. One of the things he had me do was give my 5 ghz network a different name than my 2.4 ghz network.
This didn’t fix it but has resulted in learning the trigger that cause homekit to go offline. It is the handshake from when your phone switches from your WiFi and switches to cellular.
Today I received a replacement controller. That also didn’t fix it. I set it up and the first time I left home it went offline. Sometimes it would reconnect when I got back on my WiFi but not always.
I had the controller on the 5 ghz and also my iPhone. After a few more disconnects I switched my phone to the 2.4 ghz WiFi so that I could try adding the controller to the 2.4 ghz WiFi. But got the same disconnect behavior.
But then I told my iPhone to forget the 2.4 ghz WiFi and only use the 5 ghz network. So now the rachio is no longer directly connecting to my phone when I’m home as it is on a different WiFi than my phone is.
So far success, It hasn’t disconnected and I’ve been really trying hard to get it to go offline by turning the WiFi on and off on my iPhone. Actually homekit overall seems to work slighter better like this.