In Google Assistant I have selected several news sources. One of the sources, Cnet, always plays first when I ask Google Assistant to play my news. If I remove Cnet it plays everything else in the order I have specified. If I add Cnet back in any position other than the first it will go back to playing Cnet before all else. I have yet to find a workaround.
Sorry to hear about the frustrations that you're having with the Google Assistant, I'm happy to help.
I would suggest visiting our Google Assistant Forum and writing to them there as this is a question that they would be able to help answer.
Please let me know if you have any further questions.