Google Home won't show my SmartThings devices

I designed my own custom IoT device, then programmed it using Sinric Pro and linked it to SmartThings.

It’s working great via SmartThings -> Alexa, so I know my hardware and configuration isn’t the problem. The device shows up as connected in the SmartThings app. In the SmartThings app, Voice assistant setting, both Alexa and Google Assistant correctly say “Connected to SmartThings.”

However, Google Home refuses to add SmartThings.

I followed this tutorial:

In Google Home I click “Set up Device” -> “Have something already set up?” -> select “SmartThings” -> and “Authorize”. It says for a brief moment, “Linking your SmartThings account.”… And then nothing happens. It’s not showing me any device.

All I see in Google Home is,
“You don’t have any rooms yet. Add your devices to see everything in one place.”

I’m using Android 10 on a Pixel 3. Whats going on here?