Stream URL from m3u playlist is changed by import through "IPTV Automatic Network" function and no longer usable.
When importing the m3u playlists, the underscore "_" is replaced by an HTML encoding "%5F". Therefore the created muxes do not work.
Currently all muxes have to be edited manually.
The corresponding forum post with examples can be found here: https://tvheadend.org/boards/4/topics/45090?r=45414#message-45414.