I had a problem on Friday with a new Gen 3 getting it to join on WiFi with the iPhone 11 Max running 13.4.1 and a Eero Mesh setup. I grabbed a iPad Mini 3 and was able to get it to work that way.
After that, I got bit with the Homekit bug and was playing around with the Gen 3 before actually installing it to replace my Gen 2.
One thing I noticed is the Gen 3 came with older firmware and it updated itself once it got it setup. As I messed with Homekit, I removed the Gen 3 and re-added it to the Rachio app using the iPhone and it worked fine on that attempt. Only thing that changed was the firmware update.
It would appear that the wifi joining issue is fixed with the latest firmware but getting online to pull it down is the trick I suppose. 
But then, HomeKit breaks… 