I saw your tv model and maybe 50htz will not solve the problem for you. But it worth to try.
Also check if the channel your Chromecast ultra is plugged in is running 1080p 60/120htz. And NOT 1080i or 1080p 30htz. Most TVs will display that information when you switch channel or use the button info.
Does it only happens with BG3 or did you notice that in any other game? I never played BD3 in 1080p but I could check if happens here as well later. In 4k I don't have this problem.
