It's really stupid, but on my TCL Series 6 I can no longer use the * button to call up instruction to "turn off display" I thought my remote was no longer working properly. Based on one of the answers that said a voice instruction might work I purchased a remote with voice control and was delighted to learn that when I pressed the microphone button and said "turn off display" it actually worked. And it also responded to a voice instruction to 'turn on display".
Roku should be ashamed of themselves for not disclosing this problem and the solution.