My channel have the same problem with HLS streaming (.m3u8), 2 users reported the issue with Roku Premier+ but I don't have any Roku Premier+ to troubleshoot (I tried with Roku 4, Roku stick and Express+ and they work fine). Let me know if you find out the reason.
issue fixed by setting the streamFormat
selectedItem.streamformat = "ts"
this was not needed on other roku devices.