Double-check all of the folders in:
Options > Library & Folders > Auto-Import Folders
Since each one has its own list of types, perhaps one is allowing M3U files.
i don't use auto-import, and i only have one folder in that option and it's the root directory for my music files. I've lookin' that option and the playlist was checked, so i've unchecked it. And now the m3u doesn't appear in the recently imported view, but it's imported cause it appears in > Playlists > Imported Playlists
Also, if a different playlist included a link to an M3U, this could also happen.
For what i know all my MC playlists are based on audio files not in m3u
I've to say that it's a problem because when i import a folder with one m3u file, it seems like import the songs 2 times, so when i make one general import of the folder
my music with all my music, it import hundreds of m3us and what it should take a few seconds/minutes as only should be 2 or 3 albums that wasn't imported before it can take almost
20 minutes (or more) and hundreds of new imported files ? ? ?