My 55FS3850 also just started this 2-3 weeks ago. This has to be software related. I believe the LED strips run across the tv. If it were LED backlight failure, I don't think it would just be a side that has dimmed.
If you are still experiencing this issue, can you please provide us the following information: -Roku TV brand and model -serial number -device ID -software OS/version (these can all be found in Settings > System > About) -does this issue occur on a specific channel? if so, what channel and what version/build is the channel (this can be found be selecting the channel on the Home screen and pressing the * button) -tracker ID when this issue occurs (when you see this issue occur, press the Home button 5 times, followed by the Back button 5 times) -photo/image of what you are experiencing
Once we have this information we will be able to pass it along to the appropriate team to investigate further.
Started out with intermittent blank screen. Fixed with reset of 5 times Home - 1 Up - 2 RW - 2 FF. Then ended up with a dim right side of screen; something didn't have previously. Tried everything again.