Interesting. I will always get a catchall code when I hold the button down for 3-4 seconds. They will then always stop communicating with the hub after a few hours.
The way I was finally been able to get them added was using this method:
Add a thing
Hold down button on Xiaomi device for 5 seconds to get into pairing mode
Quick press every 3 seconds
Within a minute or two of doing this, the device will show up in Add a thing. Add device normally from there.