What is the downmix setting on your Chromecast device?
Edit - guess it makes sense. As with airplay the protocol determines the options you have under the advanced settings for the device (eg my Naim that is roon ready, airplay and Chromecast - but the settings change depending on which protocol I’m using). It doesn’t look like Chromecast supports more than 2 channels for audio.
As far as we were able to determine in development and testing, Chromecast devices do not support lossless PCM multichannel output. They only support the lossy bitstream codecs commonly associated with home theater (Dolby Digital, DTS, etc).