Same issue here on the Roku Ultra using an LG TV. I have to disable HDR and then no flickering. If you switch back to HDR, no flicker. Unfortunately, when you turn off the tv and turn it back on later, the issue come back. Please fix this problem Roku, or I may have to switch to a Chromecast Ultra.
Try setting the HDR Mode to chroma subsampling of 4:2:0 instead of the default 4:2:2 in the HDMI secret screen (Home x5, Down, Left, Up x3). There is only 1 selectable field in the bottom right in this secret menu. This selectable field will either show either "HDR Mode: 4:2:2" or "HDR Mode: 4:2:0".