The problem seems to happen consistently, regardless of source file.
I know it's not a bug in the Stream Tester because the issue was first reported in our Roku app.
The problem does not happen in playlists using MPEG-TS segments. However, we need to use h.265 for 4K formats, which leaves us only with fragmented MP4 playlists. The Roku does seem to support it; it has no problem playing the video stream.