Finally solved it…
For anyone troubleshooting a similar issue in the future, try disabling minimum RSSI on all your APs temporarily. If that allows the device to connect, then check what the RSSI is to the Rachio and then reconfigure your minimum RSSI values.
For those not familiar, minimum RSSI is the minimum signal strength before the AP declines the connection request (or kicks it off, if it’s already connected). Configuring this is important for seamless roaming from AP-to-AP. If this is poorly configured then you end up staying on one AP for far too long before kicking to the other AP.
I had my minimum RSSI set to -74 dBm. Anything below -74 dBm would be kicked off the AP.
My cell phone read around -60 dBm when set on-top of the Rachio so I assumed the signal was plenty strong. However it occurred to me that the Rachio antennas are far less powerful than my iPhone.
I disabled minimum RSSI and it immediately connected. Now I can see the Rachio’s RSSI, and it’s around -81 dBm - so my minimum RSSI setting was definitely causing this issue.