I cannot tell how to connect the SA30 wireless because I prefer a wired connection. If you send music data through a wireless connection then assure the quality of the wireless connection is good, a poor quality wireless connection is prone to audio drop outs.
I always have the SA30 wired. At the time I experimented with Google Cast it worked. I don’t use Google Cast with the 867 build.
This is indeed weird. As Felix suggested please reinstall the SA30 firmware, I would recommend the USB method. Have a look at the following topic how to install the firmware. Don’t forget to factory reset and reboot the SA30. Reinstall and try Google Home with SA30 wired.