I just encountered the same issue. The flickering is occurring even on the home screen. Isolated issue to Roku. Unplugged power from Roku and it seemed stable for about 30 sec then started flickering. Just tried it again (4 hrs later), and flickering is much worse and screen now going black. Had to restart and take a series of picture with my phone to get the info below. Model 4670X, SW 9.4.0 4210-46
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.