Did not work on a 6 series TCL Roku TV. Setting changed in Netflix profile, removed netflix app, did a menu-based system restart, added Netflix back as a channel, logged in to Netflix fresh, trailer Autoplay still occurs.
I have a TCL series 6 and was experiencing the same problem. I disabled and they still played. It was driving me crazy. So I exited Netflix using the menu inside the Netflix app, not by hitting the home button or back button on the remote. I then went I, Roku to Settings-System-Power-System restart. Once it was back up I open the Netflix app up and it took a few moments longer than usual, which I interpreted as a good sign that it was reading the settings. No more preview playing, sanity restored. I hope this helps.
After trying to chat with Netflix support and not getting anywhere I came across this thread. Unfortunately Tanner's solution provided back in February did not work for me. In my case the autoplay settings were being respected for the first 30 days of my subscription. Once I went into the billing cycle the setting immediately stopped working. This issue is not occurring on my PC so it seems to be specific to the Roku Netflix app. Has there been any progress on resolving this issue?
Roku model: 4660X - Roku Ultra
Software version 9.3.0 build 4170-46
Troubleshooting steps: Re-enable/disable of autoplay settings, sign out via Netflix menu, restart of Roku, reinstall of Netflix app, switch accounts, log out of account on PC, contact Netflix support.