Hi Frances,
In the audio settings of my Google-assistant (Polk-Assist) I set the standard music player to my speaker (which is a Polk Magnify Max SR).

Then the problem occurs. If I use the Polk-Assist as my music player there is no problem at all.
The command is: Hey Google "Play Sky Radio"

I already replaced the Polk-assist with a Google Nest Mini. The same problem happens. If I cast to a Chromecast (for instance my TV) There is no problem.

The problem is that this only happens with Sky Radio and not with other streams. Maybe there is a difference in quality or bit rate or something like that.

The issue was there right from the beginning.
I hope you have enough info.