Thing is: An iOS update should have no effect on existing Rachio 3 ↔ HomeKit connectivity whatsoever. Once an iOS/IPadOS device has been used to get things set up, it need no longer even be present for the association to remain. Please see: Apple HomeKit, iOS/iPadOS, and the Rachio 3 (And other networking notes)
Is it possible there was a coincident HomePod, or whatever you’re using for a HomeKit host, OS update, too? We don’t have any HomePods, yet, but t’s been my experience that when Apple releases a major iOS/iPadOS update, there’s often a tvOS update at the same time.
If you work in I.T., then you should be aware that if you change one thing in a network environment and network behavior changes, odds are the one thing you changed is the reason for the different behavior.
This is basic Troubleshooting 101.