I have a similar issue. I have a school full of brand new Optiplex 5080 with dual R5 430. On some, no matter what I try or what webcam or usb doc cam I use, they flicker. I also have an issue where they freeze and then the main monitor goes black.
I have not found a solution. I am now using the Mother Board video DP Ports and it all works. I am very upset with this purchase. If you find a solution I hope you post it.
Somtime I can make them freeze by just plugging in a USB video device, like a webcam. Then bam, all goes black.