From the Roku Support article https://support.roku.com/article/208754908 (How to use Roku Media Player to play your videos, music and photos)...
Support for playlists
Roku Media Player supports .pls, .m3u and .m3u8 playlists on a USB drive. The media in the playlist should have an extension that indicates the media type, for example .mp3, .mkv, .jpg, etc. Playlist entries can also point to a URL. Make sure the URL begins with http:// and the extension should indicate the media type. If the playlist entry does not have a recognizable extension, it will be treated as a video and will try to detect the actual media type when it starts playing. Some DLNA servers will convert playlist files to folders that the Roku Media Player can access using DLNA.
Is the above what is occuring to you?
If not, maybe try rescanning the DLNA server (reboot and rescan), and "back-out" of the Roku Media Player (RMP) one level at a time. (Do not press Home button, use the Back button located upper left on Roku remote). Then when you go into each folder forward, this will refresh/recycle any changes you make.
I will mention that on my MiniDLNA server, I cannot see M3U or PLS files either on my RMP, so may be a setting or just a limitation of my router's DLNA. I can see M3U and PLS if I directly connect the drive to the Ultra 4660 and use RMP. Since I don't use Playlists, I really haven't explored too much why this occurs. If i really need to, I could just use another media player to connect to the drive, see the Playlists, and then send it to the TV.
If still having issues, feel free to post back. Perhaps @atc98092 can keep an eye on this thread and offer some insight for you.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941