I have a 49S405-B TCL Roku TV and there is a light horizontal line repeatedly going up the screen. This line only appears on HDMI inputs, and not the built in apps. Its only visible on dark parts of the screen. It also does not go all the way up the screen. It starts in the middle, slowly makes it way to the top, and then appears in the middle again to start again.
I have tried different HDMI ports, different HDMI devices, and changing brightness settings to hide it with no success. I didn't notice it until the TV was out of warranty.
Have you tried switching out the HDMI cable used to connect your device?
Have you tried contacting TCL support yet to inquire further? I'd suggest starting there to see what options or suggestions they can provide. Each Roku TV manufacturer provides direct support for their products running the Roku OS.
I have tried contacting them, but they just suggested I try stuff I had already tried. I have had many HDMI cables plugged into this TV. It only has HDMI and A/V, and I don't have any way to test the A/V to see if it happens for all inputs.