VLM - scene-prefix not working

https://forum.videolan.org/viewtopic.php?f=11&t=128956

I am using scene vfilter to generate thumbnails of many live stream inputs with VLM. This was/is working fine with VLC version 1.1.13, but it doesn't work with vlc 2.1.6 and above up to current version 2.2.1.

Scene-prefix is ignored and thumbnails created in /tmp/ are named just "scene.png".

VLM Config channels.conf:

del all
new test broadcast enabled loop
setup test input udp://@239.255.2.100:10001
setup test option scene-format=png
setup test option scene-ratio=5
setup test option scene-width=80
setup test option scene-height=45
setup test option scene-prefix=testchannel
control test play

Startup: $ cvlc -v -I dummy --extraintf luatelnet --telnet-password asdasd -V dummy -A dummy --video-filter scene --scene-replace --scene-path=/tmp --vlm-conf ./channels.conf

Logged into VLM's telnet interface, "show" command displays:

show
    media : ( 1 broadcast - 0 vod )
        test
            type : broadcast
            enabled : yes
            loop : yes
            inputs
                1 : udp://@239.255.2.100:10001
            output : 
            options
                scene-format=png
                scene-ratio=5
                scene-width=80
                scene-height=45
                scene-prefix=testchannel
            instances
                instance
                    name : default
                    state : playing
                    position : 0.000000
                    time : 0
                    length : 0
                    rate : 1.000000
                    title : 0
                    chapter : 0
                    can-seek : 0
                    playlistindex : 1
    schedule
>

If i use --scene-prefix=testchannel on the command line, when starting vlc 2.1.6 and above, the filename is correct.

Thank you