I’m having the same issue with the 55S20. Tired various HDMI cables and made sure that my Xbox Series X was set to HDMI 2.0 Mode in the TV input settings.
Updating on this bug. TV got a firmware update a couple days ago to version 9.4.0 build 4206-93.
Unfortunately it has not fixed this stuttering bug. Toggling game mode on and off still fixes it though, so no change.
I have the same issue - pretty annoying bug that seems like it shouldn't be difficult to fix.
I am extremely baffled almost a year without a fix
There was one update in the summer that fixed it for a month or two and then the TV had another update and they broke it again and they still haven't fixed it