Can anyone explain why my Roku Ultra blinks a white light, even with a wired connection? Also, why does Dolby Vision/HDR shut off and revert back to HD?
1) To show you its in "standby/sleeping" mode or to indicate remote response (it will eventually go out after no streaming/remote/device activity)
2) Assuming you have "HDR Always on" disabled/off, DV/HDR10/HLG modes should only be active during playback of the respectively encoded content - otherwise, output should be SDR.
(You used the term "HD", which would be relative to "4K"/"UHD", as opposed to "SDR", which would be relative to "HDR" (DV/HDR10/HLG)...)