I tried a little experiment, watching Picard the last two days, at first watched it using the app on the Sony Z9K because my Roku was unhooked ( building a new entertainment stand), hooked it back up today, noticed it was extremely brighter and more colorful using the Roku Ultra.
Kept going back and forth, app on TV vs the Roku, no question about it, much brighter and better on the Ultra.
Even on the DV Dark setting, it was still brighter, more detail and colorful using the Roku vs the app on the TV with the DV Bright setting.
Unhooked it, took it into the game room and plugged it in the the A90J OLED, did the same comparison, it was much brighter using the Roku, very noticeable difference.
Maybe it is not a TV issue, but a app issue, maybe the Android app uses a different codecs then the Roku that causes the darker picture.