We have this problem using the new MST3K app on a TCL/Roku TV – can’t use the Forward/Reverse controls. Interestingly, the Pause control works fine.
Per Joel’s request in the 3/12 Backer Newsletter, we wanted to re-watch a few minutes of the standalone version of Santos and then fast-forward to the end to get the URL provided there for troubleshooting feedback. That’s when we discovered that the Forward/Reverse controls on Roku do absolutely nothing. (And we aren’t going to watch Santos again in it’s full entirety just to get that URL to contribute troubleshooting feedback.)