The same happen to me since Juno, with Loki I could without issue. Now is not possible to upscale any video content
The only way to bypass it is to maximize it/go fullscreen on the main display and then send it to the desired one (winkey+shift+right_arrowWINKEY+SHIFT+RIGHT_ARROW)
I couldn't figure out where to watch in the logs to check what happen
--
I use chrome to watch netflix and youtube in kiosk mode
just save the app shortcut then edit it and add --kiosk and --window-position=X,0
where X stand for the x coordinate where the second display starts
https://peter.sh/experiments/chromium-command-line-switches/
check your displays with:
$ xrandr --query
that will make chrome go fullscreen and load on the second display for youtube -> youtube.com/tv (works awesome)